Final answer:
The region once a prosperous civilization that has become a barren desert due to deforestation is Mesopotamia.
Step-by-step explanation:
The region that was once a prosperous civilization but has now become a barren desert due to deforestation is Mesopotamia (D). Historically, Mesopotamia was a region of rich agricultural land and is often referred to as the cradle of civilization where early urban centers sprang up. Over time, due to various factors such as changing climate conditions and overgrazing, the desertification process enhanced, and what was once fertile land turned into desert. It's worth noting that while the Sahara Desert has undergone desertification, it was not necessarily a prosperous civilization turned barren primarily by human actions like deforestation. The desertification of the Sahara occurred over thousands of years due to climatic shifts.