The correct answer is (a) Bhagirathi.
The Bhagirathi is one of the two main tributaries of the Alaknanda River, which in turn is one of the two main tributaries of the Ganges River. The Bhagirathi River originates in the Garhwal Himalayas, near the Kedarnath Temple. It flows for about 200 kilometers before it meets the Alaknanda River at Karanprayag. The confluence of the Bhagirathi and Alaknanda rivers is considered to be the official beginning of the Ganges River.
The Pinder River is a tributary of the Alaknanda River. It originates in the Garhwal Himalayas, near the Nanda Devi National Park. It flows for about 100 kilometers before it meets the Alaknanda River at Vishnuprayag.
The Mandakini River is a tributary of the Alaknanda River. It originates in the Garhwal Himalayas, near the Kedarnath Temple. It flows for about 100 kilometers before it meets the Alaknanda River at Rudraprayag.
The Dhauli Ganga is a tributary of the Alaknanda River. It originates in the Garhwal Himalayas, near the Nanda Devi National Park. It flows for about 100 kilometers before it meets the Alaknanda River at Devprayag.