Which of the following is a vital safety device on a boiler?

A low water alarm is a critical safety device on a boiler because it serves to alert operators when the water level drops below a safe threshold. This is essential for preventing the boiler from overheating or running dry, which could lead to serious mechanical failure, explosions, or steam accidents. The low water alarm triggers an alert that allows operators to take immediate action to restore water levels, ensuring safe operation and protecting both the equipment and personnel.

In contrast, while air filters, electric fans, and pressure gauges are important components of a boiler system, they do not independently serve the primary safety functions associated with water levels. Air filters primarily deal with air quality and system efficiency rather than safety alone. Electric fans support combustion or ventilation but do not measure safety conditions directly. Pressure gauges are important for monitoring boiler pressure, but without a sufficient water level, high pressure can result in dangerous conditions. Therefore, the low water alarm is especially vital for maintaining safe boiler operation.