Final answer:
The 12 Federal Reserve Banks hold the reserves of the member banks in the Federal Reserve System. These banks support commercial banks in their districts and are integral to the U.S. monetary policy implementation.
Step-by-step explanation:
The part of the Federal Reserve System that holds reserves of the member banks is the 12 Federal Reserve Banks. These regional banks are responsible for supporting the commercial banks in their district and are involved in the implementation of the nation's monetary policy. They also hold reserves which are essentially funds that depository institutions maintain to meet regulatory requirements and to clear financial transactions. The other parts of the Federal Reserve System like the Board of Governors, the Federal Advisory Committee, and the Federal Open Market Committee (FOMC) play different roles. The FOMC, for instance, is responsible for open market operations which are a key tool used by the Federal Reserve to influence the money supply and interest rates.