Which of the following best describes national security?

National security primarily encompasses the strategies and measures taken by a nation to protect its sovereignty, interests, and citizens from external threats. This includes national defense—military readiness and capabilities to defend against attacks—and foreign relations, which involve diplomatic efforts to safeguard national interests and maintain stability. By describing national defense and foreign relations, this option captures the core components of national security, emphasizing the role of a nation in ensuring its safety and integrity on a global scale.

Cultural heritage and public health crises, while important aspects of a nation's overall well-being, do not specifically relate to the concept of national security, which is more focused on defense against external threats. Similarly, while international trade regulations may play a role in economic stability, they do not encompass the broader aspects of security related to protection from harm or conflict. Thus, the definition that highlights national defense and foreign relations stands out as the most accurate representation of national security.