Concept:Irrigation-supported cotton farming is a major pillar of the agricultural economy in some African countries.Explanation:Sudan is a leading cotton producer in Africa, and cotton growing under irrigation is a key part of its economy.Most of Sudan’s irrigated farmland depends on the Nile River and its tributaries.These waters support large-scale cotton cultivation, especially in the Gezira scheme.Zimbabwe, Mali, and Ghana are not mainly known for irrigation-based cotton as the mainstay of their agricultural economies.Answer:B. Sudan