A major river that forms a natural boundary between Mizoram and Bangladesh is the:

The correct answer is (a). The Khawthlangtuipui (Karnaphuli) is a major river that forms a natural boundary between Mizoram and Bangladesh. It originates in the Lushai Hills of Mizoram and flows through Bangladesh to the Bay of Bengal. The river is about 200 kilometers long and is a major source of water for irrigation and drinking water. It is also a popular tourist destination, with many people visiting the river to go swimming, fishing, and boating.

The other options are incorrect. The Barak is a river that flows through Assam and Bangladesh. The Dhaleshwari is a river that flows through Bangladesh. The Tlawng is a river that flows through Mizoram.