Answer:
a. desertification.
Explanation:
Desertification can be defined as encroachment of deserts to  areas that were not originally deserts which leads to loss of biological productivity. desertification is caused by some factors which are
- Climate Change
- human activities
- loss of moisture
- drought.
When the above mentioned factors  farming, overgrazing, climate change, and/or seasonal drought  set in then it changed farmland into land that cannot support plant life.
