The correct option is (C) a-iv, b-i, c-ii, d-iii
Source: Water Resources, Chapter 3
The textbook mentions Hirakud on the Mahanadi river explicitly. The other matches — Gandhi Sagar on Chambal, Nagarjuna Sagar on Krishna, and Mettur on Kaveri — are standard factual associations from the chapter's map and content. Examiners expect you to have memorised major dam–river pairs. Option (C) is the only one with all four correctly matched.