Open-end mutual funds do not offer investors a guaranteed rate of return. The performance of a mutual fund is largely dependent on the performance of the investments it holds. These investments can include a variety of assets such as stocks, bonds, and other securities. The value of these assets fluctuates with market conditions, and therefore, the return on investment from mutual funds also varies. Unlike fixed-income securities like bonds or fixed deposits, which offer a predetermined rate of return, mutual funds do not guarantee a specific return. Investors in mutual funds should be aware of this risk and invest according to their risk tolerance and investment goals.

Choice A is incorrect. Open-end mutual funds do indeed offer investors professional management. This is one of the key features of these types of funds, as they are managed by professionals who make investment decisions on behalf of the fund's shareholders.

Choice C is incorrect. Shares in open-end mutual funds are indeed redeemed at net asset value (NAV). The NAV per share is calculated daily based on the total value of all the securities in the portfolio divided by the number of shares outstanding.

Choice D is incorrect. Investors in open-end mutual funds do have flexibility to sell their holdings at will, subject to any applicable fees or charges imposed by the fund.