Concept:The Great Walls of Kano were a historical fortification built around the ancient city of Kano for security.Explanation:Kano, located in present-day Nigeria, faced threats from invading forces.To defend the city, its leaders ordered the construction of massive defensive walls.Historical records and archaeological evidence place this construction in the 15th century.This period corresponds to the 1400s CE, when Kano was growing as a major commercial and political centre.Answer:Option A: 15th century.