If volumes in a gaseous reaction are in the ratio 1:1:2, which law does this match?

Correct answer: Gay-Lussac’s law of gaseous volumes

The governing concept is Gay-Lussac’s law of combining volumes. It states that when reacting gases and gaseous products are measured at the same temperature and pressure, their volumes stand in simple whole-number ratios. The ratio 1:1:2 is exactly such a simple whole-number volume ratio, so option C is correct. The law of conservation of mass concerns total mass rather than gas volume. The law of definite proportions concerns the fixed mass composition of a pure compound, not the volumes of reacting gases. Reciprocal proportions is a different mass-ratio relationship among elements. Thus the decisive clue is the explicit gaseous volume ratio, especially its simple integer form and the implied common temperature and pressure conditions.