Step 1: Understanding RBI's headquarters.
The Reserve Bank of India (RBI) is the central banking institution of India, and its headquarters is located in Mumbai, Maharashtra.
Step 2: Analyzing the options.
(a) Mumbai: Correct. The RBI's headquarters is located in Mumbai.
(b) Delhi: Incorrect. Delhi is not the headquarters of the RBI, though it has a branch.
(c) Kolkata: Incorrect. Kolkata is home to one of the RBI's regional offices, but not the headquarters.
(d) Goa: Incorrect. Goa does not have the RBI's headquarters.
Step 3: Conclusion.
The headquarters of RBI is in Mumbai, so option (a) is the correct answer.
Final Answer:} Mumbai.