Explanation:

- Spiti River is a tributary of the Sutlej River. It flows through the Spiti Valley in the Himalayas and eventually joins the Sutlej River.

- Option 1: Chenab - The Chenab River does not have the Spiti River as a subsidiary. It is formed by the confluence of the Chandra and Bhaga rivers in the Lahaul region.

- Option 2: Ravi - The Ravi River is an entirely separate river system and is not connected to the Spiti River.

- Option 3: Sutlej - The Spiti River is indeed a tributary of the Sutlej River, making this the correct answer.

- Option 4: Beas - The Beas River also does not have the Spiti River as a tributary. It is distinct from the Spiti River system.