Final answer:
Water is the most significant agent of erosion, shaping landscapes by carving valleys, forming flood plains, and creating coastal features.
Step-by-step explanation:
The most significant agent of erosion is water. Water, as part of the hydrosphere, including rivers, glaciers, and oceans, has a tremendous impact on shaping the Earth's landscape. The processes of weathering and erosion by water increase the content of dissolved minerals, which are crucial for aquatic life. Over time, the action of water can carve out valleys, create flood plains, and form features like cliffs and caves along coastlines. While other factors such as wind, gravity, and human activities also contribute to erosion, water is the primary force due to its ubiquitous presence and the amount of material it can move.