The correct answer is B. B. N. Rau.
B. N. Rau was the Constitutional Advisor of the Constituent Assembly. He was a lawyer and diplomat who had served as the Indian High Commissioner to the United Kingdom. He was appointed as the Constitutional Advisor in 1946 and played
a key role in drafting the Constitution of India.Sachchidananda Sinha was a freedom fighter and politician who served as the President of the Constituent Assembly. He was a
member of the Indian National Congress and had been a close associate of Mahatma Gandhi.Gopalaswamy Ayyangar was a lawyer and politician who served as the Minister of Law and Justice in the first Indian government. He was a member of the Indian National Congress and had been a close associate of Jawaharlal Nehru.
H. N. Kunzru was a lawyer and politician who served as the President of the Indian Legislative Assembly. He was a member of the Liberal Party and had been a close associate of Motilal Nehru.
B. N. Rau was the most qualified person to be the Constitutional Advisor. He had a deep understanding of the Indian Constitution and was familiar with the workings of the British Parliament. He was also a skilled negotiator and was able to get the different groups in the Constituent Assembly to agree on a common draft of the Constitution.