What does a white disk on a fire hydrant indicate?

A white disk on a fire hydrant is a standardized marker used by many fire departments to indicate that the fire hydrant is out of service. This visual cue alerts firefighters and other responders that the hydrant should not be used for firefighting purposes due to maintenance issues or other concerns that may render it inoperable. Proper identification of hydrants is crucial during emergencies, as it ensures that emergency personnel can quickly locate functional hydrants and maintain effective firefighting operations.

The other options pertain to different conditions or statuses of fire hydrants but do not align with the significance of the white disk marker. For instance, low or high-pressure hydrants would be indicated by different colors or markings, not a white disk. Additionally, a designation indicating a wrong fire hydrant in service would have its own specific identifier separate from the standard markings used to show a hydrant is out of service. Thus, the use of a white disk is specifically linked to indicating that the hydrant is not operable and should not be relied upon in an emergency.