A hanging valley is a geological formation that is formed due to the action of a glacier. Glaciers are large masses of ice that slowly move and erode the landscape. As a glacier moves down a valley, it erodes the bedrock, creating a U-shaped valley. However, sometimes smaller tributary glaciers join the main glacier and erode their own valleys. These tributary valleys are usually at a higher elevation than the main valley. When the glacier retreats or melts, the tributary valleys are left hanging above the main valley, creating a hanging valley.

The other options provided in the question, namely a river, ocean, and wind, do not have the same erosive power as glaciers. While rivers can shape valleys, they do not result in the formation of hanging valleys. Similarly, the action of the ocean and wind does not lead to the formation of hanging valleys.