The correct answer is B) warn the engineer of low water level. Fusible plugs are installed in fire-tube boilers to melt and indicate a dangerously low water level. When the water level drops to the point where the top of the fire tube is exposed, the fusible plug will melt, releasing steam and alerting the engineer to add more water to the boiler. This safety feature helps prevent damage to the boiler from overheating. The other options are incorrect because they do not accurately describe the primary purpose of fusible plugs in fire-tube boilers. Fusible plugs are not used to drain the boiler (A), cool the crown sheet (C), or open electrical firing circuits (D).