Why are Dun valleys considered different from ordinary river plains?

Correct answer: They are long valleys located between mountain ranges

Dun valleys are long, relatively narrow valleys found between Himalayan mountain ranges. In the usual Indian geography description, they lie between the Shiwalik range and the Lesser Himalaya, also called the Himachal range. They are formed in a longitudinal depression between these ranges and may contain broad valley floors with settlements and cultivation. Dehradun is the best-known example of a Dun valley.

Option A is correct because it describes their position and elongated form. They are not marine coral islands, which develop in warm shallow seas. They are also not the sandy dunes of the Thar Desert, nor black-soil lava areas of the Deccan region. The important distinction is that an ordinary river plain is generally a depositional lowland formed by river processes, whereas a Dun is a structural valley enclosed between mountain ranges.