The correct answer is option 2, Bhagirathi. The Tehri dam is built on the Bhagirathi river. Bhagirathi is one of the two main headstreams of the Ganga river, the other being the Alakananda. The dam is located in the state of Uttarakhand, India. It is a multipurpose rock and earth-fill embankment dam, which serves the purpose of generating hydroelectric power, supplying water for irrigation, and preventing floods in the downstream areas. The dam is known for its immense size and is one of the tallest dams in the world. It has a significant role in the development of the region as it provides a reliable source of electricity and water for various purposes.