River valleys are mainly formed by which action?

Correct answer: By river erosion

A river valley is a low area or elongated depression formed as flowing water wears away the land. This wearing away is called river erosion. Fast water, especially in the upper course, can cut downward and sideways into soil and rock. Over a long period, the channel and its sides become broader or deeper, producing a valley. Weathering and mass movement may assist this process, but the river’s erosional action is the main process named in this question.

Option B is correct because river erosion removes and transports material from the land, gradually shaping a valley. Wind deposition does not explain the main formation of a river valley, and sea evaporation or a change in ice colour has no such land-forming role here. The important geography link is simple: flowing river water cuts the surface, so river valleys are chiefly associated with erosion.