The correct answer to the question is option 'B', which states that banks keep 15% of deposits for their day-to-day transactions. This means that when individuals or businesses deposit money in a bank, the bank sets aside a certain percentage of those deposits to meet its daily operational needs.

Here is a detailed explanation of this answer:

1. Definition of deposits:
- Deposits refer to the money that individuals or businesses entrust to banks for safekeeping and use as per their requirements.
- Banks offer various types of deposit accounts, such as savings accounts, current accounts, fixed deposit accounts, etc.

2. Purpose of keeping a portion of deposits:
- Banks need to maintain a certain level of liquidity to meet the demands of their customers for cash withdrawals, loan disbursements, and other transactions.
- To ensure that they have sufficient funds available for these operations, banks retain a portion of the deposits.

3. Reserve requirement:
- The percentage of deposits that banks are required to keep as reserves is known as the reserve requirement.
- Reserve requirements are set by the central bank or regulatory authority of a country to maintain stability in the banking system and control the money supply.
- The reserve requirement can vary from country to country and may be periodically adjusted by the central bank.

4. Calculation of the portion of deposits kept by banks:
- In this case, the correct answer states that banks keep 15% of deposits for their day-to-day transactions.
- This means that for every $100 deposited in the bank, $15 will be set aside as reserves.
- The remaining $85 can be used by the bank for lending, investment, and other operational purposes.

5. Importance of reserve requirement:
- The reserve requirement serves as a safety net for banks, ensuring that they have sufficient funds to meet their obligations.
- It also helps in maintaining the stability of the banking system by preventing excessive lending and potential bank runs.
- By controlling the reserve requirement, the central bank can influence the money supply in the economy, which in turn affects inflation and interest rates.

In conclusion, banks keep a portion of deposits for their day-to-day transactions, and in this case, the correct answer is 15%. This reserve requirement helps banks maintain liquidity, meet customer demands, and ensures the stability of the banking system.