Concept:Terracing is a soil-conservation technique, not a common farming system in West Africa.Explanation:West African agriculture commonly relies on systems such as crop rotation and rotational bush fallowing.Subsistence farming is also widely practised by many farmers in the region.Terracing, however, is mainly used on steep slopes to reduce soil erosion rather than as a standard farming system.Therefore, terracing is the practice that is not a common farming system in West Africa.Answer:C. terracing