The best explanation of an environmental effect of slash-and-burn cultivation is:
Air pollution and long-term land-cover change are side effects of slash-and-burn agriculture that have a long-ranging effect on a broader region.
This option accurately reflects the significant and widespread environmental impacts associated with slash-and-burn agriculture. While the practice may have short-term benefits for farmers in terms of soil fertility, its long-term effects include deforestation, habitat destruction, loss of biodiversity, air pollution from smoke, and changes in land cover that can affect the climate and ecosystem services over larger areas.