Answer:
Buying and selling of government securities by the Fed.
Step-by-step explanation:
Open market operations is a part of monetary policy instruments that Fed utilize in controlling the money supply in an economy.
When there is a need to reduce the money supply, then Fed should sells the government securities to the public which will reduce the money supply.
On the other hand, if there is a need to increase the money supply then as a result Fed should buy the government securities from the public and this will increases the money supply.