A

C. The interest rate.

Explanation:

Mortgage can be considered a loan that the borrower should pay to a bank for the ownership of a house. The interest rate is the price of the mortgage or a proportion of the loan that an individual should pay for getting a loan. The interest rate varies for different countries and can be up to 20%.

Usually, people take mortgages when they cannot afford to buy a house and what to pay for it within the following years. While paying such loans, individuals pay the principal, which is the price of the house, and the mortgage interest. One of the payment schemes is presented below.

Interest Vs Principal.
Example of a payment scheme (Source: www.money.stackexchange.com)