Oxidation replaces a layer of metal atoms with a layer of oxide, and whether that new oxide layer protects the metal or not depends purely on how much bigger the oxide is compared to the metal it replaced, volume for volume. That comparison, oxide volume divided by the volume of metal consumed to form it, is what the Pilling Bedworth ratio captures. A ratio below 1 leaves a porous, non protective scale, a ratio between 1 and 2 usually gives a tight, protective layer, and a ratio well above 2 builds up so much compressive stress that the scale buckles and flakes off. Since the definition is built on comparing these two volumes, the answer is volume of oxide over volume of metal, option (C).