The correct answer is (a). The Constituent Assembly was a body of elected representatives who drafted the Constitution of India. It was set up on 9 December 1946 and consisted of 389 members, including 292 elected members from the provinces, 93 members from the princely states, and 4 members nominated by the President of the Assembly. The Assembly met for the first time on 11 December 1946 and its last meeting was on 26 November 1949, when the Constitution was adopted. The Constituent Assembly was presided over by Dr. Rajendra Prasad, who later became the first President of India.
The other options are incorrect. The Governor General of India was the representative of the British Crown in India. The Parliament of India was not in existence at the time the Constitution was adopted. The British Parliament was the legislature of the United Kingdom, which had ruled India as a colony until 1947.