The correct answer is: b) British colonial rule.
Shillong was the capital of undivided Assam from 1874 to 1947, during the British colonial period. It was chosen as the capital because of its central location and its temperate climate. The city was also home to a number of British administrators and their families.
After Indian independence, Shillong became the capital of the new state of Meghalaya. It remains the capital of Meghalaya today.
The other options are incorrect. Mughal rule in Assam lasted from the 16th to the 18th centuries, but Shillong was not the capital of Assam during this time. After Indian independence, Shillong became the capital of the new state of Meghalaya, not Assam. The Ahom kingdom was a kingdom that ruled over Assam from the 13th to the 19th centuries, but Shillong was not the capital of the Ahom kingdom.