Concept:A cash crop is grown for sale and export rather than for local food use.Explanation:During the colonial period, Nigeria’s economy was structured to supply raw materials to Europe.Among the listed crops, cocoa was introduced and promoted as a major export cash crop, especially in the western region.Cocoa plantations and smallholder farms expanded rapidly because of high demand for chocolate in Europe.Rice, millet, and maize were mainly food crops, not leading export commodities at that time.Answer:D. Cocoa