Final answer:
A closed runway is marked with large X symbols to indicate it is unusable and may be hazardous, a critical safety measure for pilots.
Step-by-step explanation:
A closed runway in aviation is marked by one or more X symbols, which are large enough to be seen from the air. This marking indicates that the runway is not available for use and may be hazardous for landing or takeoff. Pilots are alerted to the runway's status through notices to airmen (NOTAMs), as well as these visual cues. It's critical for pilots to be aware of such markings to maintain safety during flight operations.