A. S.N Mukherjee (III. Chief Draughtsman):

• S.N Mukherjee served as the Chief Draughtsman during the framing of the Indian Constitution. He played a crucial role in the technical aspects of drafting and formulating the constitutional provisions.

B. Rajendra Prasad (I. President of Constituent Assembly):

• Rajendra Prasad was elected as the President of the Constituent Assembly. He presided over the sessions and discussions, maintaining order and guiding the assembly during the process of framing the constitution.

C. B.N. Rau (IV. Constitutional Advisor):

• B.N. Rau served as the Constitutional Advisor to the Constituent Assembly. He provided valuable insights and advice on constitutional matters, contributing to the drafting process.

D. B.R. Ambedkar (II. Chairman of the Drafting Committee):

• B.R. Ambedkar chaired the Drafting Committee of the Constituent Assembly. As the Chairman, he played a pivotal role in drafting the final text of the Indian Constitution and is often recognized as the principal architect of the constitution.

So, the correct option is [2].