Which of these is a role of the Department of Defense?

The role of the Department of Defense primarily revolves around the security of the United States. This encompasses a broad spectrum of responsibilities, including the protection of the nation's interests, military readiness, and the defense of the country from external threats. The department is tasked with overseeing the armed forces and ensuring that they are equipped and prepared to respond to potential dangers, thus playing a critical role in maintaining national security.

In contrast, diplomatic negotiations typically fall under the purview of the State Department, which handles foreign relations rather than military operations. Economic development is usually a focus of various agencies and organizations that specialize in commerce and economic policy. Public housing management is managed by specific governmental departments that focus on housing and urban development, which does not relate to the Defense Department's core mission. Therefore, the focus on securing the nation distinctly qualifies the Department of Defense's primary function as being centered around security.