One of the main purposes of the Fed's reserve requirements is to

A) provide cash flow for business firms.
B) provide sources for foreign exchange.
C) provide sources for foreign investments.
D) regulate the lending ability of member banks.

The main purpose of the Fed's reserve requirements is to regulate the lending ability of member banks. By requiring banks to hold a certain percentage of deposits in reserve, the Fed can control the amount of money that banks can lend out, which in turn affects the overall money supply in the economy. This is a tool used by the Fed to help stabilize the economy and prevent inflation or recession. The other choices are not related to the purpose of reserve requirements.