Step 1: Read the question.
We must find who calculates the Gross Domestic Product (GDP) in India.
Step 2: Recall the official agency.
National income figures, including GDP, are compiled by the National Statistical Office (NSO).
Step 3: Note its parent ministry.
The NSO works under the Ministry of Statistics and Programme Implementation and was formed by merging the CSO and NSSO.
Step 4: Rule out the RBI.
The Reserve Bank uses GDP data for monetary policy but does not compute it.
Step 5: Rule out the Finance Ministry and NITI Aayog.
The Finance Ministry uses GDP for budgeting and NITI Aayog gives policy advice, but neither calculates GDP.
Step 6: State the answer.
GDP is calculated by the National Statistical Office, option C.
\[ \boxed{\text{National Statistical Office (NSO)}} \]