Final answer:
A U-shaped valley is characteristic of glacial erosion, and among the options, D. Fjord correctly identifies a glacially eroded valley flooded by seawater, making it U-shaped.
Step-by-step explanation:
A U-shaped valley is indeed characteristic of glacial erosion. When a valley glacier, which is a glacier that begins at a cirque at the head of a valley or in a plateau ice cap, flows downward between the walls of a valley, it erodes the landscape to create a U-shaped valley. These valleys are distinctly different from V-shaped valleys carved by rivers, and this broad, rounded profile is a classic indication of past glacial activity.
Among the options provided (A. Arête B. Cirque C. Hanging valley D. Fjord), the correct answer is D. Fjord. A fjord is a long, deep, narrow body of water that reaches far inland, and is typically associated with where a glacial valley has been inundated by the sea. While arêtes are sharp ridges, cirques are amphitheater-like valley heads, and hanging valleys are elevated valleys that were formed by tributary glaciers, fjords match the description of U-shaped valleys that result from glacial erosion and subsequently are filled with seawater.