Which document establishes the official mission of the Marine Corps?

The National Security Act of 1947 is the correct answer because it formally established the United States Marine Corps as an integral part of the armed forces and detailed its missions and functions. This act reorganized the military establishment and set the framework for the roles of each branch, including the Marine Corps. By defining the mission within the broader context of national defense, the act ensures that the Marine Corps operates under a clear mandate aligned with national security objectives.

The other options, while significant in their own contexts, do not specifically delineate the official mission of the Marine Corps. The Constitution of the United States establishes the framework for the government and its branches, but does not specify the mission of the Marine Corps. The Department of Defense Directive provides guidelines and policies for defense operations but does not serve to define the Marine Corps' foundational mission. Finally, the Marine Corps Manual serves as a comprehensive guide on policies and procedures but it is based on the framework established by the National Security Act and does not itself create the official mission.