Final answer:
The main problem the Great Green Wall in Africa is aiming to prevent is desertification, Answer A). This ambitious project involves planting trees to stop the spread of deserts.
Step-by-step explanation:
The Great Green Wall is a project in Africa that is primarily aimed at preventing desertification. It consists of planting a vast wall of trees across the continent to combat the spread of desertification, which is the process by which fertile land becomes desert, typically as a result of drought, deforestation, or inappropriate agriculture.
The selection provided includes Answer A) Desertification, B) Eutrophication, C) Commensalism, D) None of the above, of which, A) Desertification is the correct main problem that the Great Green Wall is aiming to prevent.