The correct answer is: b) 1891.
The British formally annexed the Lushai Hills (present-day Mizoram) in 1891. The Lushai Hills were a remote and mountainous region in the northeast of India, inhabited by the Lushai people. The British had been trying to control the region for many years, but the Lushai people had resisted all attempts at conquest. In 1891, the British launched a major military campaign against the Lushai Hills. The campaign was successful, and the British were able to annex the region. The annexation of the Lushai Hills was a significant event in the history of British India. It marked the end of the Lushai people’s independence and their incorporation into the British Empire.
The other options are incorrect. Option a) is incorrect because the British did not formally annex the Lushai Hills in 1850. Option c) is incorrect because the British did not formally annex the Lushai Hills in 1935. Option d) is incorrect because the British did not formally annex the Lushai Hills in 1947.