The correct answer is B) Low water level cutoff switch. The low water level cutoff switch is a critical safety device that automatically shuts off the fuel supply to the boiler if the water level drops below a safe operating level. This helps prevent damage to the boiler and potential hazards. It is an essential component that must be tested prior to lighting off an auxiliary boiler to ensure it is functioning properly and can reliably shut down the boiler in the event of a low water condition. The other options are not directly related to the safe lighting off of the boiler. Voltage output of the ignition transformer, insulation resistance readings, and the automatic bottom blow valve, while important components, do not have the same direct impact on boiler safety during the lighting off process as the low water level cutoff switch.