Which of the following is a method used to prevent soil depletion?
a-adding fertilizers
b-planting grasses on slopes
c-dune fences

The best method used to prevent soil depletion would be contour farming. 

In contour farming, the land is tiled in consistent elevation. Contour farming is efficient in reducing soil depletion because it can conserve rain water and reduce the erosion of soil. 

If there is no contour farming in your option, then choose A. Adding fertilizer.