Which type of interest is calculated at the end of each interest period and added to the principal?

Compound interest is the type of interest that is calculated at the end of each interest period and added to the principal. This means that each subsequent interest calculation is based on the original principal plus any interest that has been added in previous periods. As a result, the amount of interest earned can grow significantly over time since you essentially earn interest on interest.

This is in contrast to simple interest, which is calculated only on the initial principal amount throughout the investment period, without considering any accumulated interest from previous periods. Annuities and perpetuities refer to payment structures rather than interest calculation methods, with annuities involving regular payments made at specific intervals and perpetuities representing income that continues indefinitely. Thus, the definition and mechanics of compound interest make it the correct answer.