The correct answer is: b) Undivided Assam.
Shillong was the capital of undivided Assam from 1874 to 1972. It was also the summer capital of British India from 1874 to 1947.
United Bengal was a province of British India that existed from 1905 to 1912. It was formed by the merger of the provinces of Bengal and Bihar and Orissa. The capital of United Bengal was Calcutta.
Greater Assam was a proposed state that would have included the present-day states of Assam, Meghalaya, Arunachal Pradesh, Nagaland, Manipur, Mizoram, and Tripura. The proposal was made in 1947, but it was not implemented.
The princely state of Tripura was a state in British India that existed from 1826 to 1949. The capital of Tripura was Agartala.