The correct answer is C) It is an inert gas. Carbon dioxide (CO2) is an inert gas, meaning it does not actively participate in chemical reactions. This property makes it an effective fire extinguishing agent, as it displaces oxygen and smothers the fire without reacting with the burning materials. Carbon dioxide is primarily used on Class B (flammable liquids) and Class C (energized electrical equipment) fires, as it does not conduct electricity and does not leave a residue. The other options are incorrect: A) Carbon dioxide is heavier than air, not lighter, and B) while it is used on Class B fires, it is not mostly used on Class A (ordinary combustible) fires.