Which river forms the boundary between Assam and Meghalaya?

The correct answer is: d) Umngot.

The Umngot River is a river in the state of Meghalaya, India. It is a tributary of the Brahmaputra River. The river originates in the Khasi Hills and flows for about 100 kilometers before joining the Brahmaputra River. The Umngot River is a popular tourist destination, known for its clear waters and white-water rafting.

The other options are incorrect because:

• Dhansiri River is a river in the state of Assam, India. It is a tributary of the Brahmaputra River. The river originates in the Naga Hills and flows for about 250 kilometers before joining the Brahmaputra River.
• Jiadhal River is a river in the state of Meghalaya, India. It is a tributary of the Umngot River. The river originates in the Khasi Hills and flows for about 50 kilometers before joining the Umngot River.
• Kulsi River is a river in the state of Assam, India. It is a tributary of the Brahmaputra River. The river originates in the Garo Hills and flows for about 100 kilometers before joining the Brahmaputra River.