The correct answer is (c). The Sankosh River forms the boundary between Assam and West Bengal. It is a tributary of the Brahmaputra River and is about 200 kilometers long. The river originates in the Himalayas and flows through the states of Assam and West Bengal before joining the Brahmaputra River. The Sankosh River is an important source of water for irrigation and drinking water. It is also a popular destination for fishing and boating.
Option (a) is incorrect because the Sankosh River does not form a boundary between Bihar and West Bengal. The two states are separated by the Ganges River.
Option (b) is incorrect because the Sankosh River does not form a boundary between Assam and Arunachal Pradesh. The two states are separated by the Brahmaputra River.
Option (d) is incorrect because the Sankosh River does not form a boundary between Bihar and Jharkhand. The two states are separated by the Ganges River.