The correct answer is C) if applied promptly. A hand portable CO2 fire extinguisher is effective on burning oil fires because the CO2 gas smothers the fire by displacing oxygen, which is necessary for combustion. However, this smothering effect is only effective if the CO2 is applied promptly to the fire, before the oil has a chance to continue burning. Applying the CO2 extinguisher promptly is key to successfully putting out an oil fire. The other answer choices are incorrect because: A) foam is not necessary for a CO2 extinguisher to be effective on an oil fire, B) preventing rekindling is not the primary mechanism by which a CO2 extinguisher works, and D) attempting low velocity fog first is not required before using a CO2 extinguisher on an oil fire.