The provisional President of the Constituent Assembly refers to the individual who held the position temporarily until a permanent president was elected. In this case, the correct answer is option 1, Dr. Sachchidananda Sinha.

Option 2, Dr. Rajendra Prasad, was elected as the permanent President of the Constituent Assembly in 1946, after Dr. Sinha`s tenure.

Option 3, Dr. B.R. Ambedkar, played a crucial role in drafting the Constitution of India as the Chairman of the Drafting Committee.

Option 4, Ft. Jawaharlal Nehru, had a significant role in the freedom struggle and served as the first Prime Minister of India, but he did not hold the position of the provisional or permanent President of the Constituent Assembly.

Therefore, the correct answer is option 1, Dr. Sachchidananda Sinha.