Who presided over the first meeting of the Constituent Assembly as its temporary President?

The correct answer is Dr Sachchidananda Sinha. When the Constituent Assembly met for the first time on 9 December 1946, Sinha, its oldest member, was made the temporary President, following the French practice of letting the senior-most member preside until a regular head is chosen.

Dr Rajendra Prasad is the classic trap: he was elected the permanent President of the Assembly on 11 December 1946, two days after the first sitting, so he did not preside over the first meeting. Jawaharlal Nehru's best-known role in the Assembly was moving the Objectives Resolution on 13 December 1946 and chairing committees such as the Union Powers Committee. G.V. Mavalankar presided when the Assembly sat as the Dominion Legislature, the country's first Parliament, not when it met to frame the Constitution. Keep the three offices apart: temporary President Sinha, President Prasad, and Mavalankar for the legislative sittings.