The correct answer is A) It forms a smothering blanket on the surface of the oil. Foam is an effective extinguishing agent for oil fires because it forms a blanket on the surface of the oil, smothering the fire by cutting off the oxygen supply. The foam also helps to cool the surface of the oil and prevent re-ignition. This makes it the most appropriate choice for extinguishing an oil fire, compared to the other options provided. Option B is incorrect because cooling the surface is a secondary effect of the foam, not the primary mechanism of extinguishment. Option C is incorrect because foam should be used alone, not in combination with a solid water stream. Option D is incorrect because the foam's primary purpose is to smother the fire, not to protect firefighters from the heat.