Desertification is

a. the natural process of desert formation caused by global climate changes.
b. accelerated by intensive agricultural activity in arid and semiarid lands.
c. caused by blowing sand from existing deserts that buries surrounding arid lands and converts them to desert.
d. caused by the rain shadows cast by mountain.

b. accelerated by intensive agricultural activity in arid and semiarid lands.

Explanation:

Desertification is said be the degradation of land caused by natural processes or human activities.  Climates variation and human activities are two main cause of desertification.

Human activities like deforestation , overgrazing and intensive agricultural activities can cause desertification in a vulnerable ecosystem like the arid and semi arid area. This intensive farming depletes the soil fertility and causes a strain to the already fragile ecosystem(arid and semi arid).

Humans over exploits the limited nutrient available in this area(semi arid and arid) through intensive farming causing an unbalance ecosystem.  The dry land are further degraded by this human activities causing desertification.

Generally, desertification is accelerated by intensive agricultural activities.

a is the answer i believe.