To control credit in the economy, the Reserve Bank of India (RBI) has a few tools at its disposal. These tools include the adjustment of the bank rate and cash reserve ratio (CRR). The correct answer to the question is option 'D', which means that the RBI can use either of these tools or a combination of both to control credit in the economy.

Here is a detailed explanation of each option:

a) Decrease Bank rate and CRR:
- Decreasing the bank rate means that the RBI lowers the rate at which it lends money to commercial banks. This reduction in the bank rate encourages banks to borrow more from the RBI, which increases the availability of credit in the economy.
- Decreasing the cash reserve ratio (CRR) means that the RBI lowers the percentage of deposits that commercial banks are required to keep with the central bank. This reduction in the CRR leads to an increase in the funds available with commercial banks for lending, thereby increasing credit availability in the economy.

b) Increase Bank rate and CRR:
- Increasing the bank rate means that the RBI raises the rate at which it lends money to commercial banks. This increase in the bank rate discourages banks from borrowing from the RBI, leading to a decrease in the availability of credit in the economy.
- Increasing the cash reserve ratio (CRR) means that the RBI raises the percentage of deposits that commercial banks are required to keep with the central bank. This increase in the CRR reduces the funds available with commercial banks for lending, resulting in a decrease in credit availability in the economy.

c) Increase Bank rate and decrease CRR:
- This combination has similar effects as option b, as it involves an increase in the bank rate and a decrease in the cash reserve ratio. It would lead to a decrease in credit availability in the economy.

d) Any of the above:
- This option means that the RBI has the flexibility to use any combination of the above tools depending on the prevailing economic conditions and its monetary policy objectives.

In conclusion, the RBI has the authority to control credit in the economy by adjusting the bank rate and cash reserve ratio, and it can choose to use either of these tools or a combination of both to achieve its desired outcome.