Final answer:
The Fed's main policy tool is open market operations, particularly open market purchases, which involve buying government bonds to adjust the money supply and interest rates for economic stability.
Step-by-step explanation:
One of the Fed's main policy tools is open market operations. This involves the central bank selling or buying Treasury bonds to influence the quantity of money and the level of interest rates. Among the options provided, the most accurate description is open market purchase, which refers to the buying of government bonds from banks, consequently increasing bank reserves and generally leading to an increase in the money supply and a decrease in interest rates, contributing to greater economic stability.