Which of the following is NOT a tributary of Ganga?

Correct — C. The Lohit belongs to the Brahmaputra system, not the Ganga system.

It rises in the eastern Himalaya, flows south-west through Arunachal Pradesh, and meets the Siang and the Dibang near Sadiya in Assam. It is from that meeting that the river is known downstream as the Brahmaputra.

The Yamuna, the Kosi and the Gandak all drain into the Ganga. The Lohit drains into a different river system altogether, which makes option (c) the exception.

Why the others are wrong

• (a)
  Yamuna — The Yamuna is the longest tributary of the Ganga, rising at the Yamunotri glacier and joining the main stream at the Sangam in Prayagraj as a right-bank tributary.
• (b)
  Kosi — The Kosi is a left-bank Himalayan tributary of the Ganga, entering it in Bihar. Its habit of shifting course across the plain has earned it the name Sorrow of Bihar.
• (d)
  Gandak — The Gandak is a left-bank tributary of the Ganga, rising in Nepal and joining the main river near Patna. SSC keys this pairing directly elsewhere in the corpus.

Northern India is drained by three Himalayan river systems — the Indus, the Ganga and the Brahmaputra — and questions of this kind test which stream feeds which.

The Ganga rises at Gangotri as the Bhagirathi, is joined by the Alaknanda at Devprayag, and gathers the Yamuna, Ramganga, Ghaghara, Gandak, Kosi and Son as it crosses the plain.

The Brahmaputra rises in Tibet as the Tsangpo, turns south through the Dihang gorge into Arunachal Pradesh, and takes the Lohit, Dibang, Subansiri, Manas and Dhansiri in Assam.

Key facts

• The Lohit joins the Siang and the Dibang in Assam, and the combined river is called the Brahmaputra.
• The Yamuna is the longest tributary of the Ganga and joins it at Prayagraj.
• The Kosi and the Gandak are left-bank tributaries of the Ganga that rise in Nepal.
• The Brahmaputra is called the Tsangpo in Tibet and the Jamuna in Bangladesh.

Common traps

• Assuming that any Himalayan river must feed the Ganga. The eastern Himalaya drains into the Brahmaputra.
• Reading NOT as which is, and answering Yamuna because it is the most familiar name on the list.
• Treating the Ganga's own headstreams, the Bhagirathi and the Alaknanda, as tributaries. They combine at Devprayag to form the Ganga itself.

No directly related past PYQ was found.