Which of the following states was formerly a Union Territory before attaining…

Before India's map of States settled into its present form, different regions reached full statehood by different constitutional routes: some were first administered directly by the Centre as Union Territories and later elevated to full States by an Act of Parliament, while others followed a different route, such as being an autonomous unit within an existing State, or a protectorate later admitted as an Associate State under a special constitutional provision.

Applying this to the north-eastern region: Manipur and Tripura were centrally-administered Union Territories from 1956. The North-Eastern Areas (Reorganisation) Act, 1971 converted the Union Territory of Manipur into the full State of Manipur with effect from 21 January 1972 -- the Act's own text describes the new State as comprising the territories that were immediately before that day comprised in the Union territory of Manipur.

By contrast, the other three options reached statehood without ever being a Union Territory:

• Meghalaya was first an autonomous State within Assam (created in 1970) and was elevated to a full State by the same 1971 Act -- from autonomous-State status, not Union Territory status.

• Sikkim was a protectorate under an Indo-Sikkim treaty, then made an Associate State of India under Article 2A by the 35th Constitutional Amendment (1974), and became a full State via the 36th Amendment (1975) -- it was never a Union Territory.

• Nagaland was carved out of the Naga Hills-Tuensang Area administered within Assam and became a full State directly under the State of Nagaland Act, 1962, with effect from 1963 -- also never a Union Territory.

Since only Manipur followed the Union-Territory-to-State route among the options offered, Manipur is the correct answer.