Which of the following is NOT a location where the flag is flown 24 hours a day by specific legal authority?

The World Trade Center is not designated as a location where the flag is flown 24 hours a day by specific legal authority. In contrast, locations such as the U.S. Capitol in Washington D.C. and the Fort McHenry National Monument are mandated by law to have the flag flown continuously, symbolizing their significance and honoring the nation. Francis Scott Key's Grave is also recognized as a site of reverence where the flag is displayed at all times.

These legal mandates are established to ensure the flag is treated with the utmost respect and to represent the ideals and sacrifices associated with these historically significant places. In contrast, while the World Trade Center is a place of remembrance and honor, it is not designated under law to have the flag flown continuously, which makes it distinct from the other locations mentioned.