Concept:The rapid population growth of West African capital cities is mainly caused by the movement of people from rural to urban areas.Explanation:Many people leave villages and small towns to settle in capital cities.They move in search of better employment, education, healthcare, and improved living standards.This continuous movement of people into cities is known as rural-urban migration.Such migration has a stronger and more lasting effect on city populations than temporary events like political rallies, sport events, or religious tourism.Answer:C. rural-urban migration