Open-market operations are when the federal reserve buys and sells securities to influence the

a. reserve requirement
b. tax rate
c. money supply

Money supply. When the federal reserve buys securities, they are taken off bank balance sheets and replaced with liquid cash which expands the money supply. When the federal reserve sells securities, the securities are traded on bank balance sheets and the cash goes back into the federal reserve accounts which takes liquid cash out of the money supply or reduces it.