The correct answer is (a). Tehri dam has been constructed on the confluence of the Alaknanda and Mandakini rivers. The Alaknanda is a tributary of the Ganges River, and the Mandakini is a tributary of the Alaknanda. The dam is located in the Garhwal Himalayas in the Indian state of Uttarakhand. It is the highest dam in India and the seventh-highest dam in the world. The dam was built to generate hydroelectric power, control floods, and provide irrigation water. It has been controversial since its construction, due to its environmental impact and displacement of people.
The other options are incorrect. Option (b) is incorrect because the Bhagirathi and Asiganga rivers do not meet at Tehri. Option (c) is incorrect because the Alaknanda and Pindar rivers do not meet at Tehri. Option (d) is incorrect because the Bhagirathi and Bhilangana rivers do not meet at Tehri.