B.N. Rau served as the Constitutional advisor to the Government of India and provided significant assistance to the Drafting Committee of the Indian Constitution. He was appointed to this role because of his legal expertise and experience in constitutional matters. B.R. Ambedkar, mentioned in the options, was the Chairman of the Drafting Committee and played a crucial role in shaping the Constitution, but B.N. Rau offered expert advice and assistance in the drafting process. This collaboration between B.R. Ambedkar and B.N. Rau was instrumental in the creation of India's Constitution.