The correct answer is the Prime Minister, who serves as the ex-officio Chairman.
The National Development Council (NDC) was set up in 1952 as the apex body for reviewing and approving the country's Five Year Plans. Along with the Prime Minister, its members include Union Cabinet Ministers, Chief Ministers of all states, and members of the Planning Commission, later NITI Aayog.
Since the Prime Minister heads the Union Cabinet, it is natural that the same person also heads the NDC.