The correct answer is B) monitor the intensity of the burner flame. A flame scanner is a safety device installed in modern boiler combustion control systems that monitors the intensity of the burner flame. It is designed to detect the presence and strength of the flame to ensure proper and safe combustion. If the flame intensity falls below a certain threshold, the flame scanner will trigger an alarm or automatically shut off the fuel supply to prevent incomplete combustion and potential hazards. The other answer choices are incorrect because they do not accurately describe the primary function of a flame scanner. A flame scanner does not monitor the stack for soot fires (A), regulate burner fuel oil pressure (C), or regulate the air flow to the furnace (D). These are separate functions typically performed by other components of the boiler combustion control system.