What is the name of the physical feature that is the longitudinal valley lying between the Lesser Himalayas and the Siwalik in India?

The correct answer is Duns.

Explanation:
Longitudinal valleys lying between the Lesser Himalayas (Himachal) and the Outer Himalayas (Siwaliks) are physical features known as Duns (or Doons) in the western region and Duars in the eastern region of India.

Key Details:
1. Formation: These flat-bottomed structural valleys were formed as a result of alluvial deposition (gravel, silt, and debris brought down by rivers) when the rising Siwalik ranges obstructed river paths.
2. Examples: Notable examples of these valleys include Dehradun (the largest dun), Kotli Dun, and Patli Dun.

Overview of Other Options:
• Terai: A marshy, damp, and thickly forested plain located south of the Bhabhar belt.
• Doab: A tract of land lying between two converging rivers (e.g., the land between the Ganges and Yamuna rivers).
• Bhabhar: A narrow, porous belt of boulder-covered land situated at the immediate foothills of the Siwaliks.