How far may the water level in an unpressurized sprinkler tank fall before a signal must be sent?

The requirement for how far the water level in an unpressurized sprinkler tank may fall before a signal is sent is established by industry standards and codes, such as those from the National Fire Protection Association (NFPA). The correct distance of 12 inches is significant because it ensures that the sprinkler system maintains adequate water supply for fire protection.

A 12-inch drop is a threshold that triggers an alert to signify that the water level is critically low, thus preventing any risk of insufficient water availability during an emergency. This level is designed to ensure prompt action can be taken to replenish the water supply, safeguarding the system's readiness to operate effectively.

In contrast, the other options represent distances that are either too high or too low compared to standard safety practices, which are designed to provide ample warning before a critical low water scenario occurs. The twelve-inch standard is widely adopted in the industry as a balance between operational safety and effective monitoring.