After the confluence of which two rivers at Devprayag is the river called the Ganga?

Correct answer: Alaknanda and Bhagirathi

At Devprayag, the Alaknanda and Bhagirathi meet, and the river downstream of this confluence is known as the Ganga. This is a key fact in the geography of the Himalayan river system and in the traditional sequence of the Panch Prayag confluences. Although both rivers have separate upper courses and tributaries, their meeting at Devprayag gives the combined stream the name Ganga. Yamuna and Tons are associated with the Yamuna system, while Son, Gandak, Ghaghara, and Kosi join the Ganga farther downstream in the plains. Since none of those pairs forms the named Devprayag confluence, option A is the only correct choice.