Dr. Rajendra Prasad was chosen as the President of the Constituent Assembly of India and later as the first President of the Republic of India for several reasons:

Leadership and Respect: Dr. Rajendra Prasad was a well-respected leader and a prominent figure in the Indian independence movement. He was a key member of the Indian National Congress and had played an active role in various aspects of the freedom struggle.

Constitutional Expertise: He was known for his legal and constitutional expertise, which made him a suitable choice to preside over the Constituent Assembly. His background in law and his understanding of constitutional matters were valuable during the drafting of the Indian Constitution.

Non-Partisan Approach: Dr. Rajendra Prasad was widely regarded as a non-partisan and impartial leader. His ability to maintain a neutral stance and mediate between different factions within the Constituent Assembly helped in building a consensus on various issues.

Unifying Figure: His appointment as the President of the Constituent Assembly and later as the first President of India helped unite the diverse groups and communities within the newly independent nation. He was seen as a unifying figure.

Personal Qualities: Dr. Rajendra Prasad was known for his humility, integrity, and commitment to public service, which earned him the trust and respect of his peers.

So, the correct option is [2].