Dr. Sachchidananda Sinha was elected as the Provisional President of the Constituent Assembly on December 9, 1946. This was a temporary position. He served as the Chairman for two days, after which Dr. Rajendra Prasad was elected as the President of the Constituent Assembly on December 11, 1946.

Here's why the other options are incorrect:

• C. Rajagopalachari: He was the last Governor-General of India and played a significant role in the Indian independence movement.

• Dr. B.R. Ambedkar: He was the Chairman of the Drafting Committee of the Constitution.

•  T.T. Krishnamachari: He was a prominent member of the Constituent Assembly and later served as the Finance Minister of India.