The Yu River forms part of the border between Manipur and:

Myanmar
Nagaland
Mizoram
Assam

The correct answer is Myanmar. The Yu River forms part of the border between Manipur and Myanmar. The Yu River is a tributary of the Chindwin River, which flows into the Irrawaddy River. The Yu River is about 200 miles long. It originates in the Manipur Hills and flows through the state of Manipur in India before crossing into Myanmar. The Yu River is a major source of water for irrigation and drinking water in Manipur. It is also a popular destination for fishing and boating.

The other options are incorrect. Nagaland is a state in northeastern India. Mizoram is a state in northeastern India. Assam is a state in northeastern India.