Planning along the natural slope of the land to reduce soil erosion is called

contour farming:

Contour farming has been practiced for centuries in parts of the world where irrigation farming is important. Although in the United States the technique was first practiced at the turn of the 19th century, straight-line planting in rows parallel to field boundaries and regardless of slopes long remained the prevalent method. Efforts by the U.S. Soil Conservation Service to promote contouring in the 1930s as an essential part of erosion control eventually led to its widespread adoption.

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.

where is the scientific experiment?