The correct answer is B) warn the engineer of low water level. Fusible plugs are installed in fire-tube boilers to melt and release steam if the water level drops too low, which could lead to overheating and damage to the boiler. When the water level drops, the fusible plug melts, allowing steam to escape and warn the engineer that the water level is dangerously low, so they can take immediate action to address the issue. The other options are incorrect because fusible plugs are not used for draining the boiler (A), cooling the crown sheet (C), or opening electrical firing circuits (D). Their primary purpose is to provide a safety mechanism to prevent catastrophic failure of the boiler due to low water levels.