What main feature distinguishes Dun valleys from ordinary river plains?

Correct answer: They are long sedimentary valleys between mountain ranges

Dun valleys are long, narrow structural valleys found between Himalayan mountain ranges. In the usual Indian geography description, they occur between the Shiwalik range and the Lesser Himalaya. Rivers and streams bring and deposit sediments in these enclosed spaces, so the valleys have a notable sedimentary or alluvial character. Dehradun is a well-known example. This combination of a long valley, mountain boundaries, and deposited sediment distinguishes a Dun valley from an ordinary open river plain. Therefore, option B is correct.

The other choices refer to unrelated landforms. Sea waves can shape coastal features, coral forms islands, and moving sand creates Thar dunes, but none of these explains the Himalayan Dun structure. The word “between” is especially important: Dun valleys are not simply flat plains anywhere beside a river; they are elongated basins lying between mountain ranges. Thus the relief setting and sediment accumulation together lead to option B.