The correct answer is A) By excluding the oxygen from the fire. Foam extinguishes an oil fire by forming a blanket over the surface of the burning oil. This blanket excludes oxygen from the fire, thereby smothering the fire and preventing it from continuing to burn. The foam also cools the oil, but the primary mechanism by which foam extinguishes an oil fire is by depriving it of the oxygen necessary to sustain combustion. The other options are incorrect because they do not accurately describe the primary mechanism by which foam extinguishes an oil fire. Removing the fuel source (option B) would not be an effective or practical method for extinguishing an oil fire. Cooling the oil below the ignition temperature (option C) and increasing the weight of the oil (option D) are not the primary ways in which foam extinguishes an oil fire.