Water erosion is usually what people think of, but ice, wind and gravity can also cause erosion.
Water erosion is usually along rivers. As the water moves quickly by it rubs on the rock and dirt, causing rivers to slowly get wider.
Ice erosion is caused by glaciers. Sometimes it's a glacier rubbing against land, and there's a lake that was made by a glacier making it's way onto land and then melting.
Wind erosion is a big problem in fields and large open plains with loose dirt. The wind just blows all the dirt away.
Gravity can cause erosion and cliffs. Sometimes along a mountain or cliff the rock won't be able to hold itself up anymore, and gravity will pull it down.
Your answer would be all of them. Wind, water, ice, and gravity.
