On which one of the following rivers is the Tehri Hydropower Complex located?

Answer: (b) Bhagirathi

Tehri dam is on the Bhagirathi, just above its confluence with the Bhilangna.

The idea

A large river system contains separately named headstreams and tributaries. A dam on a headstream should be matched to that local river, even when its water later joins a better-known main river. Trace the confluence sequence before replacing one name with another.

Bhagirathi descends from the Gangotri region → Tehri impounds its valley → Bhilangna water joins the reservoir system → Bhagirathi continues downstream → Alaknanda joins at Devprayag → Combined river is called Ganga

Why the others are wrong

(a) The Alaknanda has the Vishnuprayag and Srinagar projects.

(c) The Dhauliganga has the Tapovan–Vishnugad project.

(d) The Mandakini flows from Kedarnath.

Also know

Alaknanda receives Dhauliganga at Vishnuprayag and Mandakini at Rudraprayag. Bhilangna is a Bhagirathi tributary, not another name for Alaknanda. Tapovan–Vishnugad belongs to the Dhauliganga valley. Keeping the five prayags in order helps separate nearby hydropower sites.