The correct answer is: b) Sikkim becoming a British protectorate.
The Treaty of Tumlong was signed on 10 March 1861 between the British East India Company and the Chogyal of Sikkim. The treaty resulted in Sikkim becoming a British protectorate. Under the terms of the treaty, the Chogyal agreed to cede certain territories to the British and to allow the British to station troops in Sikkim. The British also agreed to pay an annual subsidy to the Chogyal.
The Treaty of Tumlong was a significant event in the history of Sikkim. It marked the beginning of British rule in Sikkim and led to the gradual decline of the Chogyal’s power.
Option a) is incorrect because Sikkim did not become a part of British India under the terms of the Treaty of Tumlong. Sikkim remained a separate state, but it was under British protection.
Option c) is incorrect because the Chogyal’s power actually declined after the Treaty of Tumlong. The British were now the dominant power in Sikkim and the Chogyal was forced to accept their terms.
Option d) is incorrect because Sikkim did not become a part of Nepal under the terms of the Treaty of Tumlong. Sikkim remained a separate state, but it was under British protection.