Drought and leaching have contributed to the process of , a major environmental challenge in Africa.

DESERTIFICATION.

Explanation: Drought and leaching have contributed to the process of desertification which can be defined as a process by which a geographic region becomes a barren area of land or desolate terrain, especially one with little water or vegetation. The change may result from natural changes in climate or by human activity.

Drought and leaching are natural changes although might be caused by human activities but are largely natural.

Here, desertification is an environmental challenge.

c.) slabs of ocean floor return to the mantle in subduction zones.