Which department is tasked with coordinating functions related to national security?

The Department of Defense is charged with coordinating functions related to national security, making it the correct answer. This department's primary responsibility is to oversee the nation's military and ensure its readiness to protect the United States from external threats. It encompasses various branches of the armed forces and works closely with intelligence agencies to gather information necessary for the country's defense strategy.

The other departments, while crucial for their respective functions, do not have a primary focus on national security. The Department of Health and Human Services is concerned with public health and welfare, the Department of Transportation manages the nation's transportation systems and infrastructure, and the Environmental Protection Agency focuses on environmental protection and public health standards. Each of these organizations plays a vital role in their fields, but they do not have the overarching responsibility for national security that the Department of Defense holds.