The correct answer is B) trouble in the system. In a typical fire detection control unit, an illuminated blue light would indicate that there is a trouble condition or problem in the system. This is in accordance with standard fire alarm system design and operation, where different colored indicator lights are used to convey the status of the system. Blue lights specifically are used to denote a trouble or fault condition that requires attention and troubleshooting. The other answer choices are incorrect because: A) a normal power-on condition is typically indicated by a green light, not blue; C) a fire or smoke detection would be indicated by a red light; and D) backup power status is usually shown by an amber or yellow light, not blue.