The most common cause of desertification is

The most common cause of desertification is climate change, since most of the relevant climate change that is happening right now is causing the earth's surface to heat up--leading to evaporation.

The most common cause of desertification is overgrazing because the plants are exposed in too much grazing for a long time. This kind of problem will cause to poor production of the plants and result to soil erosion and desertification.
Overgrazing can also affect the water and animals and the plants. Animals will suffer pasture shortage. Since overgrazing already affected such area, the soil fertility and organic started to lessen until there will no longer be present. This is a very serious problem.

Dall of the above all of the choices make sense