Which document provides important safety and operational information about changing conditions at airports?

The correct answer is NOTAMs, which stands for Notices to Airmen. These documents are crucial because they provide timely information about various aspects of flight operations that can affect safety. This includes notifications on changes in airport facilities, temporary hazards, runway closures, navigational aids that may be out of service, and other critical updates that pilots need to be aware of before and during flight. NOTAMs are designed to ensure that pilots have current information that could influence their flight route, make informed decisions, and maintain safety standards.

In contrast, a Flight Plan outlines the intended route and operational details of a flight but does not provide real-time updates on airport conditions. The Facility Directory contains more static information about airport facilities and services but lacks the dynamic updates provided in NOTAMs. A Pilot's Manual includes operational procedures and guidelines but does not directly inform pilots of immediate changes affecting safety at airports.