Final answer:
The Fed can adjust the reserve ratio to shape the lending ability of commercial banks.
Step-by-step explanation:
The answer to your question is B. reserve ratio. The Federal Reserve has the ability to adjust the reserve ratio, which is the percentage of deposits that banks are required to keep and not lend out. By increasing or decreasing the reserve ratio, the Fed can shape the lending ability of commercial banks. With a higher reserve ratio, banks have less money available to lend out, while a lower reserve ratio allows them to have more money available for lending.