Answer:
C. the interest rate that banks charge each other for overnight loans.
Explanation:
The Fed requires commercial banks to keep some amount of deposits as reserves in their custody. If a bank does not have the required amount for that day, they borrow from banks with excess funds. The duration of this arrangement is usually overnight. The interest rate that applies is the fed funds rate.
The Fed, which is a committee of the FOMC, determines the fed funds rate. The Fed uses the fed funds rate as a monetary policy tool. It adjusts it to drive the economy in the desired direction. Banks base their interest rates on the fed funds rate.