The agency responsible for regulating the money supply in the United States is a the U. S. Bank. b the Comptroller of the Currency. c the U. S. Treasury. d the Federal Reserve.

D). The Federal Reserve.

Explanation:

As per the question, the 'federal reserve system' is the agency that is responsible for supervising the money supply in the United States. It is stated as U.S.A's central banking system that it has a number of responsibilities to fulfill. The primary responsibilities include the supervision of financial institutions, regulating the money supply in the economy, fiscal agent of the government of the United States. etc. Therefore, it primarily works for ensuring a stable, flexible, and guarded monetary as well as the financial system. Thus, option D is the correct answer.