Option D is correct because according to the textbook, 'At Devprayag, the Bhagirathi meets the Alaknanda; hereafter, it is known as the Ganga.' This clearly states that Devprayag (Deva Prayag) is the place of confluence of the Alaknanda and the Bhagirathi rivers. The other options are incorrect: Vishnu Prayag is where the Dhauli and Vishnu Ganga meet; Rudra Prayag is where the Mandakini meets the Alaknanda; and Karan Prayag is where the Pindar joins the Alaknanda.