A river that forms a natural border between Nagaland and Assam is the:

The correct answer is: c) Disang.

The Disang is a river in India that flows through the states of Assam and Arunachal Pradesh. It is a tributary of the Brahmaputra River. The Disang River originates in the Patkai Hills in Arunachal Pradesh. It flows south through the state of Assam, where it forms a natural border between Nagaland and Assam. The Disang River then joins the Brahmaputra River near Dibrugarh in Assam.

The Disang River is a major source of water for irrigation and drinking water in Assam. The river is also a popular destination for fishing and boating.

The other options are incorrect because:

• The Dhansiri River is a river in Assam that flows through the districts of Golaghat, Jorhat, and Sivasagar. It is a tributary of the Brahmaputra River.
• The Dikhow River is a river in Assam that flows through the districts of Golaghat, Jorhat, and Sivasagar. It is a tributary of the Brahmaputra River.
• The Tizu River is a river in Nagaland that flows through the districts of Tuensang and Mon. It is a tributary of the Brahmaputra River.