Step 1: Understanding the question:
The question asks about the specific role of 'constitutional advisor' to India's Constituent Assembly, distinct from the Drafting Committee's chairman or the Assembly's president.
Step 2: Key fact:
Sir B. N. Rau (B. N. Rao) served as the Constitutional Advisor to the Constituent Assembly. He prepared the original draft of the Constitution based on Assembly debates and comparative constitutional research, which the Drafting Committee then refined.
Step 3: Why other options are wrong:
Dr. B. R. Ambedkar chaired the Drafting Committee; Dr. Rajendra Prasad was the President of the Constituent Assembly; Jawaharlal Nehru was the political leader who moved the Objectives Resolution, none of these three held the specific title 'Constitutional Advisor.'
Final Answer:
B. N. Rao (option iv).