The correct answer is A) Steel tools. Steel tools should never be used to clean boiler fuel oil atomizer parts because they can damage the soft, delicate metal components. Boiler fuel oil atomizers are typically made of brass or other softer metals that can be easily scratched or gouged by the harder steel tools. This can lead to improper sealing and leaks in the atomizer, which could potentially cause a fire or explosion hazard. Safer options for cleaning atomizer parts include using copper, brass, or wooden tools that are less likely to cause damage to the sensitive components.