I believe the answer is: c. interest rate
In mortgage we let an orgniazation (such as bank) to took ownership of a certain property that we want. We then pay the organisation with a certain amount of payment plus interest, and the ownership would be transferred to us after we complete all of the payment.
In this process, interest rate is the profit that the organization would take for their service. As the interest rate become lower, the amount of mortgage price would typically increased, and vice versa.