The Constitutional advisor to the Constituent Assembly was:

The correct answer is C) B.N. Rau. He was appointed Constitutional Adviser to the Constituent Assembly in 1946. His office prepared the background material, studied constitutions of other countries, and produced the initial draft that the Drafting Committee later took up. The Drafting Committee itself was chaired by B.R. Ambedkar, and it was this committee that piloted the draft through the Assembly.

• A) Sachidanand Sinha was the provisional chairman who presided over the first sitting on 9 December 1946, before Rajendra Prasad was elected permanent President. He was not the adviser.
• B) K.M. Munshi was a member of the Drafting Committee, not its adviser.
• D) T.T. Krishnamachari was also a Drafting Committee member, who joined after the earlier vacancy caused by the death of N. Madhava Rao's predecessor, D.P. Khaitan.

The core concept is the distinction between the Constituent Assembly's officers, its drafting committee and its expert staff. For Mains, this helps in answering questions on how the Constitution was actually drafted and on the role of bureaucratic expertise in constitution-making.

Trap: Candidates often pick Sinha because he presided over the first sitting, or Munshi and Krishnamachari because they were prominent drafting members. Remember that the adviser is a separate post from both the presiding officer and the drafters.