Step 1: Office-bearers of the Assembly.
Dr. Rajendra Prasad was elected {permanent President} of the Constituent Assembly. (Dr. Sachchidananda Sinha acted as the {temporary} President at the first meeting.)
Step 2: Clarify roles of others.
Dr. Ambedkar chaired the Drafting Committee; Nehru moved landmark resolutions (Objectives Resolution); Gandhi was not a member of the Assembly.
Step 3: Conclude.
\[
\boxed{\text{Dr. Rajendra Prasad (Option A).}}
\]