How does an open market operation influence the monetary aggregates?

How does an open market operation influence the monetary aggregates? 

Ans. When there is a need to expand credit, central bank buys securities which increases availability of funds with the banks for lending purpose and when there is a need to contract credit, central bank sells securities which decreases availability of funds with the banks for lending purpose. In other words, central bank will buy treasury bills if it wishes to increase money supply and sell treasury bills if it wants to reduce it. An increase in money supply will make balance sheet of private sector more liquid.