Answer: The federal reserve uses open market operation to buy government bonds from the public,by doing this they reduce the federal fund rate.
Open market operation is a situation were central bank gives or takes liquidity currency to a bank or group of banks. It is use to implement monetary policy. Federal government buy and sell government securities to control money supply.
The federal government sells government bonds,so that they can raise the federal fund rate,this reduces the quality of reserve in the banking system,which increase the need for bank to borrow,thereby making the federal fund rate to rise.