Answer : 2. "Dr.Rajendra Prasad "
Explanation :

The Constituent Assembly held its first meeting on Dec.9,1946. The meeting was attended by 211 members. Dr. Sachchidanand Sinha, the oldest one,was elected as the temporary president of the assembly. Later on, Dec.11,1946, Dr. Rajendra Prasad and H.C. Mukherjee were elected as the President and vice-president of the assembly.Sir B N Rau was appointed as the Constitutional Advisor in the assembly.