monetary base  in a country is the total amount of bank notes and coins, total currency held by the public,plus the currency held in the vaults of commercial banks, and  commercial banks' reserves held in the central bank. if they purchase  government bonds from commercial banks, central bank  pay for these banks  by adding new amounts to the banks’ reserve deposits at the central

.Central bank purchases  bonds from commercial banks, the central bank takes the  illiquid bonds and replace it with a cash deposit in the bank’s reserve .

So commercial banks' reserves held in the central bank is a component of monetary base .So eventually the monetary base will increase.

Option c-Decrease in money supply is not correct.When central bank purchase government securities or bonds it will increase money supply not decrease the money supply.When government purchases securities from public .It will increase cash holdings of a person .

Option D- Cause reserve ratios to decrease is not  correct .The central bank wants to increase money supply .It is using one method that is it is increasing open market purchases.Reducing reserve ratio is another method to control money supply in the economy .The central bank will not use two methods together to control money supply in the economy.Currently they are using open market operations or purchases .Maybe in other situation ,they will use this method to control money supply in market