Sikkim was merged with India in:

May 1975

The correct answer is Option 3 - May 1975

Sikkim was merged with India in May 1975

The first democratic elections to Sikkim assembly in 1974 were swept by Sikkim Congress which stood for greater integration with India. The assembly first sought the status of ‘associate state’ and then in April 1975 passed a resolution asking for full integration with India. This was followed by a hurriedly organised referendum that put a stamp of popular approval on the assembly’s request. The Indian Parliament accepted this request immediately and Sikkim became the 22nd State of the Indian union (in May 1975). Chogyal did not accept this merger and his supporters accused the government of India of foul play and use of force. Yet the merger enjoyed popular support and did not become a divisive issue in Sikkim’s politics.

Note: The date of integration of Sikkim (i.e. May 1975) with India is not given in NCERT.