Final answer:
The impact that occurs when a desert expands into previously arable areas is desertification, which leads to a loss of fertile land, decrease in food production, and potentially a famine. (option a)
Step-by-step explanation:
When a desert expands into areas that were previously arable, the impact is desertification. Desertification refers to the process of land turning into desert, typically due to various factors such as climate change, overgrazing, deforestation, and improper agricultural practices. This expansion of desert areas leads to a loss of fertile land, making it unsuitable for agriculture and leading to a decrease in food production, ultimately resulting in a famine.
Additionally, desertification can also have other consequences such as the destruction of habitats, loss of biodiversity, and socio-economic challenges for affected communities.