The permanent President of the constituent assembly was

Correct option is B

The permanent President of the Constituent Assembly was Dr. Rajendra Prasad.

Supporting Information
Dr. Rajendra Prasad was unanimously elected as the permanent President of the Constituent Assembly on December 11, 1946.

Dr. Sachchidananda Sinha served as the interim (temporary) President for the first meeting only; the role of permanent President was then taken over by Dr. Rajendra Prasad.

The other listed individuals had important but distinct roles: Dr. B. R. Ambedkar was Chairman of the Drafting Committee; Jawaharlal Nehru chaired other important committees; K. M. Munshi was involved in procedural committees, but not as President.

Hence, the correct answer is Dr. Rajendra Prasad.