The correct answer is: a) A popular referendum in 1975
The Chogyal Palden Thondup Namgyal was the last hereditary ruler of Sikkim, a Himalayan kingdom that was annexed by India in 1975. In 1974, the Sikkim State Congress, a political party that advocated for Sikkim’s merger with India, won a majority of seats in the Sikkim Legislative Assembly. The Chogyal refused to accept the results of the election, and in 1975, India intervened militarily and forced him to abdicate. A popular referendum was then held, in which the majority of Sikkimese voters approved of Sikkim’s merger with India.
Option b) is incorrect because India did not intervene militarily in Sikkim until 1975, after the Chogyal had already been deposed.
Option c) is incorrect because China never invaded Sikkim.
Option d) is incorrect because the Sikkim State Congress was a political party that advocated for Sikkim’s merger with India, not for a rebellion against the Chogyal.