Step 1: Adoption date and authority.
The Constituent Assembly adopted the Constitution on 26 November 1949. Certain provisions came into force immediately; the rest commenced on 26 January 1950 (Republic Day).
Step 2: Why not the others?
Governor General/President merely promulgated or gave effect to constitutional arrangements; Parliament did not exist before commencement. The act of {adoption} was the Assembly’s sovereign function.
Step 3: Conclude.
\[
\boxed{\text{Adopted by the Constituent Assembly (Option A).}}
\]