Final answer:
To increase the money supply by $50 billion, the Fed should buy $50 billion worth of U.S. government bonds.
Step-by-step explanation:
To increase the money supply by $50 billion, the Fed should buy $50 billion worth of U.S. government bonds. When the Fed buys bonds, it increases the reserves of the banks it buys from. Since the reserve requirement ratio is 10%, banks are required to hold 10% of their deposits as reserves. So when the reserves increase, the banks can lend out more money, which increases the money supply.