Concept:U-shaped valleys are carved by glacial erosion. A glacier's slow movement and great weight reshape a V-shaped river valley into a wide, steep-sided U-shaped trough.Explanation:Glaciers erode the valley floor and sides uniformly, creating a broad, U-shaped cross-section. This shape offers the least friction for the moving ice. The glacier straightens the valley and makes its walls nearly vertical. Striations (scratches) from dragged boulders are common. The valley floor often contains till or moraines. In contrast, rivers carve V-shaped valleys, wind creates desert landforms, and sea waves form coastal features like cliffs and beaches.Answer:Option A. glacier is correct.