The Constituent Assembly adopted our National Anthem on

India's National Anthem and National Song were both given their status at a single sitting of the Constituent Assembly — an act distinct from when the Constitution itself was adopted (26th November, 1949) or came into force (26th January, 1950). The trap in this question is assuming the Assembly transacted National-Anthem-related business on every date offered, or confusing it with these other constitutional milestones.

On 24th January, 1950 — the Constituent Assembly's last sitting — its President, Dr. Rajendra Prasad, formally declared "Jana Gana Mana" the National Anthem of India; the same sitting gave "Vande Mataram" equal status as the National Song and saw Dr. Prasad elected the first President of India, two days before India became a sovereign democratic Republic.

What the other dates offered actually correspond to:

• 20th January, 1950 — no Constituent Assembly sitting is recorded for this date.

• 21st May, 1949 — this fell on a Saturday, a few days after the Assembly's mid-May sitting that debated and adopted the resolution ratifying India's continued membership of the Commonwealth of Nations (16th-17th May, 1949).

• 13th November, 1949 — this was a Sunday, immediately before the Assembly's final round of debate on the draft Constitution, which ran from 14th to 26th November, 1949.

The correct milestone is 24th January, 1950.