Which department is chiefly involved in national defense matters?

The Department of Defense is the primary agency responsible for coordinating national defense and military policies for the United States. It oversees the armed forces and is tasked with ensuring the security of the nation against external threats. This department is involved in a wide range of activities, including military operations, resource allocation for defense, intelligence operations, and interactions with other defense agencies.

In contrast, the other departments listed are focused on different sectors; the Department of Health and Human Services deals with public health and welfare programs, the Environmental Protection Agency is concerned with environmental protection and regulation, and the Food and Drug Administration is focused on food safety, pharmaceuticals, and medical devices. Each of these departments has a distinct mission that does not relate to national defense, which underscores why the Department of Defense is the correct choice in this context.