Answer:
(c) the Sahel
Step-by-step explanation:
You want to know which area the map shows as very high risk of desertification.
Desertification
Land will generally be transformed to desert if it is an area of low rainfall, and land use is such that ground-covering plants are stressed. Usually, such areas are already on the borders of existing deserts.
The map shows the Sahara and Kalahari are already deserts. The Congo River is an area of abundant rainfall and water resources, so is shown as Humid/Not vulnerable.
The Sahel is on the southern border of the Sahara, so has a very high vulnerability to desertification.