How are closed runways marked?

A closed runway is marked with a white "X" to indicate to pilots that the runway is not in use and should not be landed upon or taken off from. This marking is vital for safety, ensuring that pilots can quickly identify that the runway is closed for operations.

The choice of a white "X" is significant because it stands out against the runway surface, making it easily visible. The use of white is also consistent with other airport signage where white is commonly employed for important information.

The other options do not represent standard practices for indicating closed runways. For instance, a red circle or a yellow "X" are not recognized markings for closed runways, and while a solid line might suggest a boundary, it lacks the immediate recognition factor required for runway closure notifications. Thus, the clear and unmistakable white "X" serves its purpose effectively in maintaining safe operations at airports.