Which river joins the Alaknanda at Rudraprayag?

Correct answer: Mandakini

The governing concept is the location of major confluences in the Alaknanda basin. At Rudraprayag, the Mandakini River joins the Alaknanda, making option A correct. Rudraprayag is one of the Panch Prayag, the important confluence points traditionally used to understand the upper Ganga drainage system. Nandakini joins the Alaknanda at Nandprayag, while Kali and Saryu are separate Himalayan rivers and are not the river pair identified with Rudraprayag. The best method is to learn each prayag together with its tributary: Vishnuprayag–Dhauliganga, Nandprayag–Nandakini, Karnaprayag–Pindar, Rudraprayag–Mandakini, and Devprayag–Bhagirathi.