The correct answer is: c) 1901.
The British formally annexed the Naga Hills region into Assam in 1901. This was done after the British had defeated the Nagas in a series of wars. The British wanted to control the Naga Hills because they were a strategic location and because they were rich in natural resources. The annexation of the Naga Hills led to a lot of resentment among the Nagas, who felt that they had been conquered and their way of life was being destroyed. This resentment eventually led to the Naga Insurgency, which began in 1956.
Option a) is incorrect because the British East India Company signed a treaty with the Ahom kingdom of Assam in 1826, which gave the British control of the Brahmaputra Valley. However, the Naga Hills were not included in this treaty.
Option b) is incorrect because the British did not formally annex the Naga Hills until 1901.
Option d) is incorrect because the British did not formally annex the Naga Hills until 1901.