What does the NAS include?

The National Airspace System (NAS) encompasses a comprehensive framework that includes U.S. navigation facilities, which are critical for the safe and efficient operation of aircraft within U.S. airspace. This system integrates various elements such as airports, air traffic control facilities, navigational aids, and communication systems, along with all associated personnel who ensure the smooth operation of air traffic management.

B is the correct choice because it accurately describes the NAS as a holistic system comprising navigation facilities and all relevant components and personnel that contribute to airspace operations. This integration is essential for maintaining safety, efficiency, and the overall functionality of aviation in the United States.

The other options do not fully capture the scope of the NAS. For example, stating that it includes only U.S. airports overlooks the importance of navigation facilities and personnel, while referencing only military airspace or international navigation facilities fails to represent the entirety of the NAS, which is specifically designed to serve both civil and military aviation across the nation.