The correct answer is: a) Sikkim becoming a British protectorate.
The Treaty of Tumlong was signed on 18 September 1861 between the British East India Company and the Kingdom of Sikkim. The treaty resulted in Sikkim becoming a British protectorate. The treaty was signed after the British had invaded Sikkim in 1861. The British were concerned about the growing influence of the Tibetans in Sikkim and wanted to ensure that Sikkim remained under British control. The treaty gave the British control over Sikkim’s foreign affairs and defense. The treaty also allowed the British to station troops in Sikkim. The treaty was a major blow to Sikkim’s sovereignty.
Option b) is incorrect because Sikkim did not lose any territory to the British as a result of the Treaty of Tumlong.
Option c) is incorrect because the Treaty of Tumlong did not result in the end of the Namgyal Dynasty. The Namgyal Dynasty ruled Sikkim from 1642 to 1975. The Treaty of Tumlong did not affect the succession of the Namgyal Dynasty.
Option d) is incorrect because the Treaty of Tumlong did not result in full independence for Sikkim. Sikkim became a British protectorate as a result of the Treaty of Tumlong.