Which one of the following rivers is NOT the part of Indian Ganga river basin?

The question tests where the Chhotanagpur plateau drainage goes. The Ganga basin in eastern India is fed from the plateau's northern and eastern edges, while the Subarnarekha and Brahmani-Baitarani systems drain southeast towards the Bay of Bengal.

• Punpun (A) rises in the Chhotanagpur plateau and flows north-east into the Ganga near Patna. It is part of the Ganga basin.
• Ajoy (B) flows through Jharkhand and West Bengal and joins the Bhagirathi. It is part of the Ganga basin.
• Jalangi (C) is a distributary-type channel of the Bhagirathi-Hooghly system in Nadia, West Bengal, so it belongs to the Ganga delta.
• Jonk (D) is the one that does not drain into the Ganga. It flows through the plateau towards the southeast, outside the Ganga system.

The core concept is river basin delimitation. Ganga-basin rivers are identified by their confluence with the Ganga or its major tributaries. Mains often asks about basin-level issues such as sediment load, the Farakka-related water disputes, and inter-state water sharing in the Jharkhand-Bihar-West Bengal belt.

Note that the exact course and confluence of smaller Jharkhand rivers are not always covered in standard textbooks, so verify against the state's river map when you revise.

Trap: Aspirants pick Ajoy or Jalangi because they sound like Bengal rivers. Bengal drainage is not always Ganga drainage, and the Subarnarekha and Damodar systems sit beside it.