Extinguishing an APU fire is achieved with which of the following?

The correct method for extinguishing an APU fire is through the single halon fire bottle dedicated to the APU. Halon is an effective fire suppression agent specifically designed to extinguish fires by disrupting the chemical reaction of combustion. In many aircraft systems, including the CRJ550, the APU is equipped with its own dedicated halon fire bottle that can be activated from the flight deck or manually in the rear compartment where the APU is located. This system is crucial as it provides a quick and efficient means to combat an APU fire without the need for additional equipment or personnel to respond.

Other options, while they may seem plausible, do not specifically align with the standard procedures and equipment used for APU fire suppression in modern aircraft. For instance, backup water systems and foam-based firefighting systems are not typically designed for use with an APU fire due to the specific requirements for extinguishing such a fire safely and effectively. Manual fire extinguishers, while useful in many situations, do not provide the specialized response tailored for an APU fire scenario and may lack the effectiveness of halon, which is engineered for aviation use.