What type of payment structure involves consistent additions over time?

The correct choice of an annuity stems from the definition and characteristics of this financial structure. An annuity is a series of equal payments made at regular intervals over a specified period of time. This can occur annually, semi-annually, quarterly, or monthly. The defining feature is the consistency in the payment amounts, which provides predictability for both the payer and the payee.

In contrast, perpetuity refers to a payment structure that continues indefinitely, with no end date, and typically consists of recurring payments of the same amount forever. While it involves consistent payments, it does not fit the "over time" qualifier in terms of having a defined duration.

Regular rate and payment plan, while they might imply some form of payment structure, do not specifically denote the concept of equal periodic payments over time in the way that an annuity does. A regular rate could refer to a fixed interest rate but does not pertain directly to payments. Meanwhile, a payment plan can vary widely in its payment schedule and amounts, and might not involve consistent additions over time.

Thus, the definition and structure of an annuity align perfectly with the idea of consistent additions, making it the most accurate choice.