Final answer:
Unsustainable agricultural practices such as tilling, slash-and-burn farming, and overgrazing can cause desertification, which is the degradation of land in arid, semi-arid, and dry sub-humid areas, leading to the loss of topsoil, nutrients, and fertility. Therefore, the correct option is D.
Step-by-step explanation:
Unsustainable agricultural practices such as tilling, slash-and-burn farming, and overgrazing can cause a decrease in the productivity of the land. Specifically, they can lead to desertification, which is the process whereby fertile land becomes desert as a result of various factors, including these harmful agricultural practices. This environmental issue results in the loss of topsoil, reduction in the availability of fertile land for farming, and can aggravate food scarcity and biodiversity loss.
Slash-and-burn agriculture is particularly destructive as it depletes soil nutrients rapidly, leading to the abandonment of the land. Overgrazing by livestock can result in soil compaction and loss of vegetation, both of which can also contribute to desertification. These actions accelerate soil erosion and nutrient depletion, leaving the land unable to support plant life. Desertification is a major concern globally, with substantial areas of once-productive land turning into barren landscapes.