The correct answer is: a) Nepal.
Sikkim was invaded and occupied by Nepal in the 18th century. The Gurkhas, a Nepalese ethnic group, invaded Sikkim in 1772 and captured the capital, Tumlong. The Sikkimese were forced to sign a treaty in 1774 that made Sikkim a tributary of Nepal. The Gurkhas ruled Sikkim for over 50 years. In 1814, the British East India Company went to war with Nepal. The British defeated the Gurkhas and forced them to withdraw from Sikkim. Sikkim became a British protectorate in 1817.
Option b) Bhutan is incorrect because Bhutan did not invade Sikkim in the 18th century. Bhutan and Sikkim were allies at the time.
Option c) The British East India Company is incorrect because the British East India Company did not invade Sikkim in the 18th century. The British East India Company did not become involved in Sikkim until the 19th century.
Option d) China is incorrect because China did not invade Sikkim in the 18th century. China had a claim to Sikkim, but it did not attempt to enforce its claim until the 19th century.