Explanation:

- The financial sector in India is regulated by various institutions to ensure its stability and integrity.

- Option 1: RBI (Reserve Bank of India)

- It is the central bank of India.

- It regulates the banking system and oversees monetary policy.

- Option 2: SBI (State Bank of India)

- It is a commercial bank and not a regulatory body.

- Option 3: IDBI (Industrial Development Bank of India)

- It is now a commercial bank, initially set up as a development finance institution.

- Option 4: None of the above

- Incorrect, as RBI is indeed the regulator.

Correct Answer: RBI