The correct answer is option 2, Dr. Rajendra Prasad. He was the Chairman of the Constituent Assembly of India. The Constituent Assembly was established in 1946 to draft the Constitution of India. Dr. Rajendra Prasad, who later became the first President of India, was elected as the Chairman by the members of the Constituent Assembly.

Option 1, Dr. B.R. Ambedkar, was a prominent member of the Constituent Assembly and played a crucial role in drafting the Constitution as the chairman of the Drafting Committee.

Option 3, Jawaharlal Nehru, was the first Prime Minister of independent India, but he was not the chairman of the Constituent Assembly.

Option 4, Vallabh Bhai Patel, also known as Sardar Patel, was another prominent member of the Constituent Assembly, but he was not the chairman.

Therefore, the correct answer is indeed option 2.