Final answer:
Open-market operations (Option b) are a monetary policy instrument used by central banks to influence the economy's money supply, interest rates, and aggregate demand. The correct option is b.
Step-by-step explanation:
The question asks to identify which of the listed options is a monetary policy instrument. Option (b), open-market operations, is indeed a tool used by a central bank to conduct monetary policy. A central bank, such as the Federal Reserve, executes monetary policy using three traditional tools.
These are: open market operations, reserve requirements, and discount rates. Open market operations involve buying and selling government bonds with banks and are the most commonly used tool.
This action affects the supply of money and loanable funds in the economy, which in turn influences interest rates, borrowing behaviors, as well as investment and consumption patterns, ultimately impacting aggregate demand and GDP. The correct option is b.