Who was the Chairman of the Constituent Assembly?

Who was the Chairman of the Constituent Assembly? [SSC Combined Matric Level Exam 21/05/2000, Shift I], [SSC Combined Matric Level Exam 24/10/1999, Shift II]

          a) Dr. B.R. Ambedkar

          b) Dr. Rajendra Prasad

          c) Jawahar Lal Nehru

          d) Vallabh Bhai Patel