The longest and biggest river in Nagaland is the:

The Doyang river is the longest and largest river of Nagaland. It flows mainly through the Wokha and Zunheboto districts and is a tributary of the Dhansiri river, which in turn joins the Brahmaputra in Assam.

Course of the Doyang:

• It rises in the southern part of Nagaland, near the Japfu range area south of Kohima.
• It flows northwards through central Nagaland, cutting deep valleys through the hills.
• It then turns west and enters Assam, where it joins the Dhansiri river.

Doyang Hydroelectric Project:

• A hydroelectric project with a capacity of 75 MW was built on the Doyang river in Wokha district by NEEPCO (North Eastern Electric Power Corporation).
• Its reservoir, the Doyang Reservoir, has become an important wetland.

Amur falcon conservation:

The Doyang Reservoir area in Wokha district is famous for the huge gatherings of Amur falcons, small birds of prey that stop over in Nagaland every October-November on their long migration from Siberia and northern China to southern Africa.

In 2012, large-scale hunting of these birds was reported. After awareness campaigns by local communities, NGOs and the government, the Pangti village area has become a model of conservation, and Nagaland is called the "Falcon Capital of the World".

Other rivers of Nagaland:

• Dhansiri: flows along the western border and drains into the Brahmaputra.
• Dikhu, Jhanji and Tsurang: other rivers of the Brahmaputra system.
• Tizu: flows eastward into Myanmar and joins the Chindwin river.

About Nagaland:

• It became the 16th state of India on 1 December 1963.
• Its capital is Kohima, and its largest city is Dimapur.
• The Hornbill Festival, held every December at Kisama near Kohima, celebrates Naga culture.

Why not the other options?

• ✗The Dhansiri, Dikhu and Tizu are rivers of the region, but the Doyang is the longest and largest within Nagaland.