Which of the following refers to a decline in the fertility of land and quality of soil?

Land degradation

Land degradation is the deterioration or loss of the productive capacity of the soils for present and future. 

Ozone depletion refers to the depletion of ozone in the layer which is found in "stratosphere".

Deforestation is the purposeful clearing of forested land.

Global warming refers to increase in the average tempurature of the planet.