Answer: D) Flat area with a large number of plants
The roots of the plants hold in the soil, which prevents/reduces erosion.
The steeper slope means gravity pulls down the soil material faster, since there is less ground in the way to hold it up, so to speak. So flatter areas are less prone to erosion compared to steeper areas.
So that's why overall, flatter areas with lots of plants would have the least erosion.