Final answer:
The countries Nigeria, Mali, and Liberia are all located in West Africa, a region known for its involvement in the trans-Atlantic slave trade and the development of yam-based agriculture.
Step-by-step explanation:
The countries listed in the question - Nigeria, Mali, and Liberia - are all located in a particular region of Africa. According to historical and geographical records, the region most directly involved in the Trans-Atlantic slave trade and which independently developed agriculture based on yams was West Africa. This region stretches from modern-day Mauritania to the Democratic Republic of the Congo. It encompasses diverse ecological zones such as lush rainforests, savannas, and drier lands. The historical African empires of Ghana, Mali, and Songhay were also located in West Africa, with Great Zimbabwe being the exception as it was not located in this region.