The correct answer is: a) Lack of democratic representation.
The people of Sikkim were unhappy with the Chogyal because they did not have any say in how their country was run. The Chogyal was an absolute ruler, and he made all the decisions without consulting the people. This led to a lot of resentment, and eventually the people revolted and overthrew the Chogyal.
The other options are also possible grievances, but they were not as major as the lack of democratic representation. Economic inequality and exploitation were problems in Sikkim, but they were not as severe as in some other countries. Restrictions on religious freedom were also a problem, but they were not as severe as in some other countries. Suppression of cultural identity was not a major problem in Sikkim.
In conclusion, the people of Sikkim were unhappy with the Chogyal because they did not have any say in how their country was run. This led to a lot of resentment, and eventually the people revolted and overthrew the Chogyal.