What does an "Open Door" sign indicate at ATC facilities?

An "Open Door" sign at ATC facilities indicates that the facility is open for business. This sign communicates to both staff and the public that the operations are functioning normally and that individuals can enter freely without any restrictions. In the context of air traffic control and aviation services, clear communication about the facility's operational status is crucial for maintaining safety and efficiency.

The other options may suggest various operational statuses like maintenance or emergency drills, which would typically be signified using different indicators or announcements, rather than an "Open Door" sign. Thus, the correct understanding of this signage is essential for anyone interacting with ATC facilities.