Soil erosion can be prevented by (a) raising forests.
Explanation: Planting trees (afforestation) helps to hold the soil in place with their roots, preventing erosion caused by wind and rain.
Why the other options are incorrect:
(b) deforestation: Deforestation is the removal of trees, which exposes the soil to erosion.
(c) excessive use of fertilizer: While excessive use of fertilizers can have negative environmental impacts, it is not the primary cause of soil erosion.
(d) overgrazing by animals: When animals graze excessively, they remove vegetation cover, leaving the soil exposed to erosion.