All the following are commonly used in agriculture and pollute the soil EXCEPT O A.
Erosion.
OB.
Herbicides.
O C.
Fertilizer.
OD
Pesticides.

C. Fertilizer

Explanation:

Fertilizer is used to help plants grow, and plants are fragile. So, it wouldn't pollute soil. If it did, it would most likely be banned.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.