What differentiates a flexible premium annuity from other annuities?

A flexible premium annuity is characterized by its ability to accept varying contributions over time, unlike other types of annuities that often require fixed premium payments. Specifically, a flexible premium annuity does not promise a specific amount for future benefits at the outset; instead, the benefits depend on the total contributions made and the performance of the underlying investments up until the time benefits are paid out. This feature allows for adaptability in funding the annuity based on the policyholder's financial situation.

By contrast, the option referring to a one-time payment describes a single premium annuity, where the entire investment is made at once. The choice about immediate payouts relates to immediate annuities, which begin payments almost right after purchase. Lastly, an option that guarantees a fixed amount regardless of contributions pertains to fixed annuities, which provide defined benefits without variation from investment performance. Thus, the defining feature of a flexible premium annuity lies in its lack of predetermined future benefits, making option B the correct answer.