When did Tripura become a Constitutional State of India?

Concept

Full statehood for a Union Territory means it is raised to the status of a constituent State under a specific Act of Parliament, with its own legislature and the powers of a State under the Constitution. For the north-eastern region this elevation was governed by the North-Eastern Areas (Reorganisation) Act, 1971.

Application

Tripura, earlier a Part-C State and then a Union Territory, was upgraded to a full-fledged State of the Indian Union when the North-Eastern Areas (Reorganisation) Act, 1971 came into force on 21 January 1972. From that day Tripura functioned as a constituent State with its own Legislative Assembly. Manipur and Meghalaya attained statehood on the same date under the same Act.

Cross-check & result

The official date is 21 January 1972. Comparing this with the listed dates — 2 April 1977, 24 March 1973 and 21 January 1975 — not one of them is 21 January 1972.

Since none of the listed dates corresponds to Tripura's statehood, the correct choice is that none of them applies.