Gandak River is a tributary of which river system?

Correct answer: Ganga system

Direct answer: Option D, Gandak is a tributary of the Ganga river system. Gandak rises in the Himalayan region, flows through Nepal and northern Bihar, and joins the Ganga near Sonpur. Step by step: identify Gandak as a Himalayan river; trace its general flow into the northern plains; observe that it meets the Ganga; conclude that its drainage belongs to the Ganga system. Option A, Narmada system, is wrong because Narmada is a separate west-flowing peninsular river. Option B, Godavari system, is wrong because Godavari drains much of peninsular India and Gandak does not join it. Option C, Tapi system, is wrong for the same geographical reason: Tapi is a separate west-flowing river. Option D is correct because Gandak directly contributes water to the Ganga. Its association with northern Bihar is a useful location clue, but the decisive fact is its connection with the Ganga. Memory cue: Himalayan tributary in north Bihar—Gandak joins Ganga.