The correct answer is (a) B.N. Rau.
B.N. Rau was a constitutional expert who was appointed as the Constitutional Advisor to the Constituent Assembly of India in 1946. He was responsible for drafting the Constitution of India, which was adopted on 26 November 1949.
K.M. Munshi was a freedom fighter and politician who was a member of the Constituent Assembly of India. He was not the Constitutional Advisor to the Constituent Assembly.
D.P. Khaitan was a lawyer and politician who was a member of the Constituent Assembly of India. He was not the Constitutional Advisor to the Constituent Assembly.
A.K. Ayya was a lawyer and politician who was a member of the Constituent Assembly of India. He was not the Constitutional Advisor to the Constituent Assembly.