Answer:
A policy instrument (variable directly under the control of policy makers)
Step-by-step explanation:
The Fed's discount rate is a monetary policy tool used to expand or contract the money supply.
When the Fed lowers the discount rate, it is engaging in an expansionary monetary policy which will increase the money supply, lower interest rates and increase total aggregate demand.
When the Fed raises the discount rate, it is engaging in a contractionary monetary policy which will decrease the money supply, increase interest rates and fight rising inflation.