Concept:Open market operation is the central bank's buying and selling of securities to regulate the money supply in the economy.
Explanation:The central bank employs several monetary policy instruments to control the amount of money in circulation.
One of these instruments involves the purchase and sale of securities, such as treasury bills and government bonds.
When the central bank buys securities from commercial banks or the public, it releases money into the economy, thereby increasing the money supply.
When the central bank sells securities, it absorbs money from the economy, thereby reducing the money supply.
This process of buying and selling securities is called open market operation.
Bank credit refers to loans and advances given by commercial banks to borrowers.
Cash reserve ratio is the proportion of customer deposits that banks must keep with the central bank.
Bank rate is the rate at which the central bank discounts bills or lends to commercial banks.
Since the question specifically refers to the instrument by which the central bank buys and sells securities, the open market operation is the correct choice.
Answer:A. Open market operation