The Law of Conservation of Mass, established by Lavoisier, states that the total mass of reactants equals the total mass of products in a chemical reaction — matter is neither created nor destroyed. Applying this law: mass of iron (+) mass of oxygen (=) mass of iron oxide. Therefore, mass of oxygen (=) mass of iron oxide (-) mass of iron (= 15.0 - 10.5 = 4.5) g. Option 3.0 g is incorrect; it is the difference between 15.0 and 12.0, which does not correspond to any quantity given. Option 5.5 g would imply the product mass is 16.0 g, not 15.0 g. Option 6.0 g would imply (\text{Fe} + \text{O}_2) masses of (10.5 + 6.0 = 16.5) g, contradicting the given product mass. This question tests the NCERT fundamental law of conservation of mass, which underpins all stoichiometric calculations. Plausibility check: the amount of oxygen (4.5 g) is a reasonable fraction of the iron mass (10.5 g), consistent with the formula (\text{Fe}_2\text{O}_3) where oxygen accounts for (48/160 = 30%) of the mass, and (4.5/15.0 = 30%).