The correct answer is: a) Nepal.
In the 18th century, Sikkim faced invasions from Nepal. The Gurkhas, a warrior people from Nepal, invaded Sikkim in 1772 and 1788. The first invasion was successful, and the Gurkhas captured the Sikkimese capital of Tumlong. The second invasion was also successful, and the Gurkhas captured the entire country of Sikkim. The Gurkhas ruled Sikkim for 50 years, until they were driven out by the British in 1814.
The British East India Company was a trading company that was founded in 1600. The company was granted a monopoly on trade with India by the British government. The company became very powerful and eventually became the government of India. The company did not invade Sikkim in the 18th century.
Bhutan is a country located in the Himalayas. Bhutan is a Buddhist country, and the government is based on Buddhist principles. Bhutan has a long history of independence, and it has never been invaded by another country.
China is a country located in East Asia. China is a very large country, and it has a long history. China has been invaded by many countries in the past, but it has never been invaded by Nepal.