Answer: money supply curve to the left.
Step-by-step explanation:
The sale of Treasury Bills by the Fed is part of its contractionary monetary policy and is used to reduce the amount of money in the economy. By selling the bills, people pay money to the Fed which then takes the money out of circulation thereby reducing the amount of money in the economy.
The effect of this would be a leftward shift in the money supply curve to indicate that there is now less money in the economy. The Fed does this when it feels that the economy is overheated and so economic growth needs to be reduced to a more sustainable level.