The most accurate statement regarding interest from the options provided is:
It is a percentage of the amount of the principal you have not yet paid back.
Interest is typically calculated on the remaining principal balance of a loan, which decreases as you make payments. The other statements are either misleading or incorrect. Interest can be calculated multiple times throughout the life of a loan (e.g., monthly, annually), and while it can be expressed as an APR (annual percentage rate), this doesn't encompass the entire picture of how interest accrues on loans.