ITEM 1.    REPORTS TO STOCKHOLDERS.

DEC    ^12.31.19

ANNUAL REPORT

AB FLEXFEE^TM HIGH YIELD PORTFOLIO

Beginning January 1, 2021, as permitted by new regulations adopted by the Securities and Exchange Commission, the Fund’s annual and semi-annual shareholder reports will no longer be sent by mail, unless you specifically request paper copies of the reports. Instead, the reports will be made available on a website, and you will be notified by mail each time a report is posted and provided with a website address to access the report.

If you already elected to receive shareholder reports electronically, you will not be affected by this change and you need not take any action. You may elect to receive shareholder reports and other communications from the Fund electronically at any time by contacting your financial intermediary (such as a broker-dealer or bank) or, if you are a direct investor, by calling the Fund at (800) 221 5672.

You may elect to receive all future reports in paper form free of charge. If you invest through a financial intermediary, you can contact your financial intermediary to request that you continue to receive paper copies of your shareholder reports; if you invest directly with the Fund, you can call the Fund at (800) 221 5672. Your election to receive reports in paper form will apply to all funds held in your account with your financial intermediary or, if you invest directly, to all AB Mutual Funds you hold.

Investors should consider the investment objectives, risks, charges and expenses of the Fund carefully before investing. For copies of our prospectus or summary prospectus, which contain this and other information, visit us online at www.abfunds.com or contact your AB representative. Please read the prospectus and/or summary prospectus carefully before investing.

This shareholder report must be preceded or accompanied by the Fund’s prospectus for individuals who are not current shareholders of the Fund.

You may obtain a description of the Fund’s proxy voting policies and procedures, and information regarding how the Fund voted proxies relating to portfolio securities during the most recent 12-month period ended June 30, without charge. Simply visit AB’s website at www.abfunds.com, or go to the Securities and Exchange Commission’s (the “Commission”) website at www.sec.gov, or call AB at (800) 227 4618.

The Fund files its complete schedule of portfolio holdings with the Commission for the first and third quarters of each fiscal year as an exhibit to its reports on Form N-PORT. The Fund’s Form N-PORT reports are available on the Commission’s website at www.sec.gov. The Fund’s Forms N-PORT may also be reviewed and copied at the Commission’s Public Reference Room in Washington, DC; information on the operation of the Public Reference Room may be obtained by calling (800) SEC 0330. AB publishes full portfolio holdings for the Fund monthly at www.abfunds.com.

AllianceBernstein Investments, Inc. (ABI) is the distributor of the AB family of mutual funds. ABI is a member of FINRA and is an affiliate of AllianceBernstein L.P., the Adviser of the funds.

The [A/B] logo is a registered service mark of AllianceBernstein and AllianceBernstein^® is a registered service mark used by permission of the owner, AllianceBernstein L.P.

FROM THE PRESIDENT

Dear Shareholder,

We are pleased to provide this report for AB FlexFee High Yield Portfolio (the “Fund”). Please review the discussion of Fund performance, the market conditions during the reporting period and the Fund’s investment strategy.

As always, AB strives to keep clients ahead of what’s next by:

Whether you’re an individual investor or a multi-billion-dollar institution, we put knowledge and experience to work for you.

AB’s global research organization connects and collaborates across platforms and teams to deliver impactful insights and innovative products. Better insights lead to better opportunities—anywhere in the world.

For additional information about AB’s range of products and shareholder resources, please log on to www.abfunds.com.

Thank you for your investment in the AB Mutual Funds.

Sincerely,

Robert M. Keith

President and Chief Executive Officer, AB Mutual Funds

ANNUAL REPORT

February 12, 2020

This report provides management’s discussion of fund performance for AB FlexFee High Yield Portfolio for the annual reporting period ended December 31, 2019.

The Fund’s investment objective is to seek to maximize total return consistent with prudent investment management.

Please keep in mind that high, double-digit returns are highly unusual and cannot be sustained. Investors should also be aware that these returns were primarily achieved during favorable market conditions.

INVESTMENT RESULTS

The table above shows the Fund’s performance compared to its benchmark, the Markit iBoxx USD Liquid High Yield Index, for the six- and 12-month periods ended December 31, 2019.

The Fund outperformed the benchmark for both periods. The Fund’s performance-based advisory fee was accrued at its minimum rate for the performance period from January 1, 2019 through December 31, 2019.

In the 12-month period, industry allocation was the primary contributor, relative to the benchmark, due to exposure to the industrials—other and energy sectors, achieved in part through the use of derivatives, including total return swaps and high-yield credit default swaps. Losses within transportation, as well as media in the communications sector, partially offset these gains. Security selection was also positive, mainly from selection within the consumer cyclical—other, basic and capital goods sectors that offset losses in energy and consumer cyclical—autos. Yield-curve positioning was a slight detractor and currency decisions did not have a material impact on overall performance.

In the six-month period, security selection was the primary contributor because of selection in basic, technology and consumer cyclical—other,

which more than offset losses within energy and consumer cyclical—autos. Industry selection also contributed, due to several modestly beneficial positions, achieved in part through the use of total return swap derivatives. Yield-curve positioning was a minor detractor, while currency decisions did not have a material impact on overall performance.

During both periods, the Fund utilized derivatives in the form of futures to hedge duration and interest rate risk and interest rate swaps to hedge duration risk. Currency forwards were used to hedge foreign currency exposure and to take active currency risk. Credit default swaps, both single name and index, were used to take active exposure as well as to hedge investment-grade and high-yield credit risk taken through cash bonds. Total return swaps were used to create synthetic high-yield exposure. Written and purchased swaptions were used to manage and/or to actively take yield-curve positions. Purchased and written equity options were used to take active exposure, while purchased currency options were used to hedge foreign currency exposure.

MARKET REVIEW AND INVESTMENT STRATEGY

Global fixed-income markets performed strongly over the 12-month period ended December 31, 2019. The US Federal Reserve lowered interest rates three times beginning in July, and increased its balance sheet (i.e., purchased assets) later in the period to manage liquidity in the repurchase agreement market, effectively capping short-term rates. The European Central Bank reduced rates to a record low in September and announced the resumption of quantitative easing (i.e., purchasing government securities to encourage lending and investment). The Bank of Japan issued guidance for the continuation of low rates and the government implemented a significant fiscal stimulus program in December. Central bankers in numerous other developed and emerging markets lowered interest rates and signaled potential fiscal stimulus measures to boost economic growth. Investor confidence increased after the announcement of a partial US-China phase-one trade agreement and indications that the UK would leave the European Union with a clear path for Brexit.

Long-dated developed-market treasuries were strong performers, given their interest-rate sensitivity, despite the increase in yields since September. Investment-grade, high-yield and emerging-market sovereign debt all posted solid returns as credit spreads tightened. The US dollar was persistently firm as a safe haven currency until the fourth quarter, when renewed optimism led investors to increase growth (risk) investments. Overall, the US dollar was mixed relative to developed- and emerging-market currencies, gaining versus the euro and Latin American currencies, while falling against the pound, Swiss franc and yen. Inflation remained below target in most developed countries and fell in emerging markets, even as oil prices rebounded.

The Fund’s Senior Investment Management Team (the “Team”) continues to seek to maximize total return, utilizing a high-yield strategy with a global, multi-sector approach. The Team invests in corporate bonds from US and non-US issuers and government bonds from developed and emerging markets, primarily focusing on lower-rated bonds (“junk bonds”), although it may also invest in investment-grade bonds.

INVESTMENT POLICIES

At least 80% of the Fund’s net assets will, under normal circumstances, be invested in fixed-income securities rated Ba1 or lower by Moody’s Investors Service or BB+ or lower by S&P Global Ratings or Fitch Ratings (commonly known as “junk bonds”), unrated securities considered by the Adviser to be of comparable quality, and related derivatives. The Fund may invest in fixed-income securities with a range of maturities from short- to long-term. The Fund may also invest in equity securities.

In selecting securities for purchase or sale by the Fund, the Adviser attempts to take advantage of inefficiencies that it believes exist in the global debt markets. These inefficiencies arise from investor behavior, market complexity, and the investment limitations to which investors are subject. The Adviser combines quantitative analysis with fundamental credit and economic research in seeking to exploit these inefficiencies.

The Fund will most often invest in securities of US issuers, but may also purchase fixed-income securities of foreign issuers, including securities denominated in foreign currencies and securities of emerging-market issuers. The Adviser may or may not hedge any foreign currency exposure through the use of currency-related derivatives.

The Fund expects to use derivatives, such as options, futures contracts, forwards and swaps, to a significant extent. Derivatives may provide a more efficient and economical exposure to market segments than direct investments, and may also be a more efficient way to alter the Fund’s exposure. The Fund may, for example, use credit default and interest rate swaps to gain exposure to the fixed-income markets or particular fixed-income securities and, as noted above, may use currency-related derivatives. The Adviser may use derivatives to effectively leverage the Fund by creating aggregate market exposure substantially in excess of the Fund’s net assets.

DISCLOSURES AND RISKS

Benchmark Disclosure

The Markit iBoxx USD Liquid High Yield Index is unmanaged and does not reflect fees and expenses associated with the active management of a mutual fund portfolio. The Markit iBoxx USD Liquid High Yield Index consists of USD high-yield bond issues with more than $400 million outstanding, selected to provide a balanced representation of the broad USD high-yield liquid corporate bond universe. An investor cannot invest directly in an index or average, and their results are not indicative of the performance for any specific investment, including the Fund.

A Word About Risk

Market Risk: The value of the Fund’s assets will fluctuate as the bond or stock market fluctuates. The value of its investments may decline, sometimes rapidly and unpredictably, simply because of economic changes or other events that affect large portions of the market.

Interest-Rate Risk: Changes in interest rates will affect the value of investments in fixed-income securities. When interest rates rise, the value of existing investments in fixed-income securities tends to fall and this decrease in value may not be offset by higher income from new investments. Interest-rate risk is generally greater for fixed-income securities with longer maturities or durations.

Credit Risk: An issuer or guarantor of a fixed-income security, or the counterparty to a derivatives or other contract, may be unable or unwilling to make timely payments of interest or principal, or to otherwise honor its obligations. The issuer or guarantor may default, causing a loss of the full principal amount of a security and accrued interest. The degree of risk for a particular security may be reflected in its credit rating. There is the possibility that the credit rating of a fixed-income security may be downgraded after purchase, which may adversely affect the value of the security.

Below Investment-Grade Securities Risk: Investments in fixed-income securities with lower ratings (commonly known as “junk bonds”) are subject to a higher probability that an issuer will default or fail to meet its payment obligations. These securities may be subject to greater price volatility, due to such factors as specific corporate developments, negative perceptions of the junk bond market generally and may be more difficult to trade or dispose of than other types of securities.

Duration Risk: Duration is a measure that relates the expected price volatility of a fixed-income security to changes in interest rates. The duration of a fixed-income security may be shorter than or equal to full maturity of a fixed-income security. Fixed-income securities with longer durations have more risk and will decrease in price as interest rates rise.

DISCLOSURES AND RISKS (continued)

Inflation Risk: This is the risk that the value of assets or income from investments will be less in the future as inflation decreases the value of money. As inflation increases, the value of the Fund’s assets can decline as can the value of the Fund’s distributions. This risk is significantly greater if the Fund invests a significant portion of its assets in fixed-income securities with longer maturities.

Derivatives Risk: Derivatives may be difficult to price or unwind and leveraged so that small changes may produce disproportionate losses for the Fund. Derivatives may also be subject to counterparty risk to a greater degree than more traditional investments.

Leverage Risk: To the extent the Fund uses leveraging techniques, its net asset value (“NAV”) may be more volatile because leverage tends to exaggerate the effect of changes in interest rates and any increase or decrease in the value of the Fund’s investments.

Foreign (Non-US) Risk: Investments in securities of non-US issuers may involve more risk than those of US issuers. These securities may fluctuate more widely in price and may be more difficult to trade or dispose of due to adverse market, economic, political, regulatory or other factors.

Emerging-Market Risk: Investments in emerging-market countries may have more risk because the markets are less developed and less liquid, and because these investments may be subject to increased economic, political, regulatory or other uncertainties.

Currency Risk: Fluctuations in currency exchange rates may negatively affect the value of the Fund’s investments or reduce its returns.

Illiquid Investments Risk: Illiquid investments risk exists when certain investments are or become difficult to purchase or sell. Difficulty in selling such investments may result in sales at disadvantageous prices affecting the value of your investment in the Fund. Causes of illiquid investments risk may include low trading volumes, large positions and heavy redemptions of Fund shares. Over recent years illiquid investments risk has also increased because the capacity of dealers in the secondary market for fixed-income securities to make markets in these securities has decreased, even as the overall bond market has grown significantly, due to, among other things, structural changes, additional regulatory requirements and capital and risk restraints that have led to reduced inventories. Illiquid investments risk may be higher in a rising interest-rate environment, when the value and liquidity of fixed-income securities generally decline.

Management Risk: The Fund is subject to management risk because it is an actively managed investment fund. The Adviser will apply its investment

DISCLOSURES AND RISKS (continued)

techniques and risk analyses in making investment decisions, but there is no guarantee that its techniques will produce the intended results.

These risks are fully discussed in the Fund’s prospectus. As with all investments, you may lose money by investing in the Fund.

An Important Note About Historical Performance

The investment return and principal value of an investment in the Fund will fluctuate, so that shares, when redeemed, may be worth more or less than their original cost. Performance shown in this report represents past performance and does not guarantee future results. Current performance may be lower or higher than the performance information shown. You may obtain performance information current to the most recent month-end by visiting www.abfunds.com.

Performance information prior to July 26, 2016 shown in this report reflects the historical performance of the AB High-Yield Portfolio, a series of The AB Pooling Portfolios (the “Accounting Survivor”), adjusted to reflect the expense ratio of Advisor Class shares of the Fund as of July 26, 2016. Upon completion of a reorganization of the Accounting Survivor into the Fund on July 26, 2016, the Fund assumed the performance and financial history of the Accounting Survivor. At the time of the reorganization, the Accounting Survivor and the Fund had substantially similar investment objectives and strategies.

Effective February 26, 2018, the Fund implemented a performance-based, or fulcrum, advisory fee. Accordingly, performance information shown prior to February 26, 2018 does not reflect performance fee adjustments and would have been different if the Fund had been managed under the performance (fulcrum) fee arrangement. Effective February 26, 2018, all previously offered shares of the Fund, including Class Z shares, were converted to Advisor Class shares.

All fees and expenses related to the operation of the Fund have been deducted. Performance assumes reinvestment of distributions and does not account for taxes.

HISTORICAL PERFORMANCE

GROWTH OF A $10,000 INVESTMENT IN THE FUND (unaudited)

12/31/2009 TO 12/31/2019

This chart illustrates the total value of an assumed $10,000 investment in AB FlexFee High Yield Portfolio Advisor Class shares (from 12/31/2009 to 12/31/2019) as compared to the performance of the Fund’s benchmark.

The Fund’s current prospectus fee table shows the Fund’s total annual operating expense ratio as 3.26% for Advisor Class shares, gross of any fee waivers or expense reimbursements. Contractual fee waivers and/or expense reimbursements limit the Fund’s annual operating expense ratio exclusive of the Fund’s advisory fees, acquired fund fees and expenses other than the advisory fees of any AB mutual funds in which the Fund may invest, interest expense, taxes, extraordinary expenses, and brokerage commissions and other transaction costs to 0.10% for Advisor Class shares. These waivers/reimbursements may not be terminated before April 30, 2020. Any fees waived and expenses borne by the Adviser may be reimbursed by the Fund until the end of the third fiscal year after the fiscal period in which the fee was waived or the expense was borne, provided that no reimbursement payment will be made that would cause the Fund’s Total Other Expenses to exceed the expense limitation. Absent reimbursements or waivers, performance would have been lower. The Financial Highlights section of this report sets forth expense ratio data for the current reporting period; the expense ratio shown above may differ from the expense ratio in the Financial Highlights section since they are based on different time periods.

EXPENSE EXAMPLE

(unaudited)

As a shareholder of the Fund, you incur two types of costs: (1) transaction costs, including sales charges (loads) on purchase payments, contingent deferred sales charges on redemptions and (2) ongoing costs, including management fees; distribution (12b-1) fees; and other Fund expenses. This example is intended to help you understand your ongoing costs (in dollars) of investing in the Fund and to compare these costs with the ongoing costs of investing in other mutual funds.

The Example is based on an investment of $1,000 invested at the beginning of the period and held for the entire period as indicated below.

Actual Expenses

The first line of the table below provides information about actual account values and actual expenses. You may use the information in this line, together with the amount you invested, to estimate the expenses that you paid over the period. Simply divide your account value by $1,000 (for example, an $8,600 account value divided by $1,000 = 8.6), then multiply the result by the number in the first line under the heading entitled “Expenses Paid During Period” to estimate the expenses you paid on your account during this period.

Hypothetical Example for Comparison Purposes

The second line of the table below provides information about hypothetical account values and hypothetical expenses based on the Fund’s actual expense ratio and an assumed annual rate of return of 5% before expenses, which is not the Fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in the Fund and other funds by comparing this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of other funds.

EXPENSE EXAMPLE (continued)

PORTFOLIO SUMMARY

December 31, 2019 (unaudited)

PORTFOLIO STATISTICS

Net Assets ($mil): $40.2

PORTFOLIO OF INVESTMENTS

December 31, 2019