Manipur, Meghalaya and Tripura became states under _____________.

The correct option is North-Eastern Areas (Reorganisation) Act, 1971.

This historical and administrative change can be understood through the following details:
1. Historical Context: Before 1972, the north-eastern region of India underwent major political and administrative restructuring to meet the aspirations of the people in the region. Manipur and Tripura were Union Territories, while Meghalaya was an autonomous state within the state of Assam.
2. The Reorganisation Act: To grant full statehood to these areas, the Parliament of India enacted the North-Eastern Areas (Reorganisation) Act, 1971.
3. Implementation: This Act came into effect on January 21, 1972, officially elevating Manipur, Meghalaya, and Tripura to the status of full-fledged states of the Indian Union. It also created the Union Territories of Mizoram and Arunachal Pradesh (which later became states).