Answer : 1. "Brahmaputra"
Explanation :

1. Brahmaputra is the widest river in India.

2. The Brahmaputra River originates as Siang or Dihang from the Chemayungdung glacier of the Kailash range near Lake Manasarovar.

3. It enters India from the west of Sadiya town in Arunachal Pradesh.

Tributaries: Dibang, Lohit, Siang, Budhi Dihing, Tista and Dhansiri.

4. The average width of the Brahmaputra valley is about 80 kilometres.

5. The main river of the valley, the Brahmaputra is one of the largest rivers in the world and ranks fifth in terms of its average runoff.

6. Namcha Barwa is a mountain peak in the Tibetan Himalayas with an altitude of 7757 meters. Tsangpo - The Brahmaputra river takes a U-turn on reaching Namcha Barwa and enters India in Arunachal Pradesh through a narrow valley.

7. The Himalayas extend in the east-west direction from the Indus Valley in the west to the Brahmaputra Valley in the east.