The correct answer is - flat surfaces.
The flat surfaces are the safest ground where the mass movements of soil and other material are least likely to appear. This is due to the flatness itself. Every mass movement is going downwards, and that is a result of the gravitational pull, but on the flat surfaces the gravitational pull functions as a protector of the soil because it is all flat, and it is pulling downwards, thus the soil and the other material are stable in the place where they are and will for sure not be moving sideways.
