The suggested answer is D.
Methane is classified under Safety Group A3, which indicates high flammability, according to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards. In contrast, Octaflouropropane and Difluoromethane are less flammable, and Methyl Chloride, while being flammable, does not have the highest flammability compared to Methane.