The best response to explain how the relationships among different regional, social, ethnic, and racial groups have impacted national identities in Africa since 1945 is:
The history of social identities in Africa has hindered the development of national identity for people of every ethnic and racial group, due to the marginalization of many people during and after colonial rule.
This statement captures the essence of how colonial legacies have led to divisions and challenges in forming a cohesive national identity in many African countries, as marginalized groups often struggle to assert their identity within a broader national framework.