Which river meets the Alaknanda at Vishnuprayag?

Correct answer: Dhauliganga

The governing concept is the identification of tributary confluences in the Alaknanda river system. At Vishnuprayag in Uttarakhand, the Alaknanda is joined by the Dhauliganga River, so option B is correct. This confluence is one of the traditionally recognised Panch Prayag, or five important meeting points, associated with the Alaknanda. Mandakini joins at Rudraprayag, Pindar joins at Karnaprayag, and Nandakini joins at Nandprayag. Those rivers are therefore plausible distractors because each is associated with another Alaknanda confluence. Remembering the river-pair rather than only the place name gives a reliable way to solve the question.