Answer : 3. "by afforestation "
Explanation :

Soil erosion is a serious environmental problem. Steps should be taken to curb this problem. Following are some ways to prevent soil erosion:

1. Plant trees on barren lands to prevent soil erosion.

2. Add mulch and rocks to trap plants and grass beneath to prevent soil erosion.

3. Mulch matting can be used to reduce erosion on slopes.

4. Place a series of fiber logs to prevent any water or soil from flowing away.

5. A wall at the base of a slope can help prevent soil erosion.

6. Every house should have proper drainage system so that water can flow into proper water collection systems.

7. Afforestation is the planting of trees on a large scale to check soil erosion.