The correct answer is option 'A', which states that banks keep 15% of deposits as cash. This is known as the cash reserve ratio (CRR), which is a policy tool used by central banks to control the money supply in an economy.

The cash reserve ratio is the percentage of deposits that banks are required to keep with the central bank as a reserve. It is a crucial tool used by central banks to maintain stability in the financial system and control inflation. By regulating the amount of cash that banks must keep on hand, central banks can influence the lending capacity of commercial banks and ultimately control the money supply.

Now, let's understand why the correct answer is option 'A' in more detail:

1. Cash Reserve Ratio (CRR):
- The cash reserve ratio (CRR) is the percentage of deposits that banks are required to keep with the central bank.
- It is a statutory requirement imposed by the central bank on commercial banks.
- The purpose of CRR is to ensure that banks have enough cash to meet their obligations and maintain liquidity.

2. Role of the Central Bank:
- The central bank is responsible for regulating the banking system and controlling the money supply.
- By adjusting the cash reserve ratio, the central bank can influence the lending capacity of banks.
- If the central bank increases the CRR, banks are required to keep a higher percentage of their deposits as cash, reducing their lending capacity.
- Conversely, if the central bank decreases the CRR, banks can keep a lower percentage of their deposits as cash, increasing their lending capacity.

3. Significance of the Cash Reserve Ratio:
- The cash reserve ratio plays a vital role in controlling inflation and maintaining financial stability.
- By regulating the amount of cash that banks can lend, the central bank can control the money supply in the economy.
- If the central bank wants to reduce inflation, it can increase the CRR, reducing the amount of money available for lending and reducing aggregate demand.
- On the other hand, if the central bank wants to stimulate economic growth, it can decrease the CRR, allowing banks to lend more money and increasing aggregate demand.

In conclusion, the correct answer is option 'A' because banks are required to keep 15% of their deposits as cash under the cash reserve ratio (CRR) policy imposed by the central bank. This policy ensures that banks have enough liquidity to meet their obligations and helps the central bank control the money supply in the economy.