Which of the following should be checked regularly on a steam boiler?

Regularly checking the water level on a steam boiler is crucial for several reasons. The water level indicates whether there is enough water in the boiler to safely generate steam. If the water level drops too low, it can lead to overheating of the boiler metal, which may cause catastrophic failures such as boiler failure or even explosions. Maintaining the correct water level is essential for the safe and efficient operation of the boiler, as it ensures that a proper steam supply is generated while preventing damage to the equipment.

The other options, while important in their respective contexts, do not require the same level of regular monitoring as the water level. Flame color can indicate combustion efficiency and safety; however, this is usually part of more specific operational checks rather than routine monitoring. Fuel type is important during initial setup or when changing supplies but isn't a variable that needs regular checks in ongoing operations. The size of the expansion tank could be relevant for system design but doesn't require frequent checks after installation.