In which law is simple volume ratio used instead of mass?

Correct answer: Law of gaseous volumes

The direct answer is option A: the law of gaseous volumes. This law, associated with Gay-Lussac, states that when gases react, their volumes can be in simple whole-number ratios when measured at the same temperature and pressure. The key word is volume, not mass. Option A is correct because it directly describes simple volume ratios of reacting gases. Option B, the law of definite proportions, says that a pure compound always contains the same elements in the same fixed mass ratio; it concerns composition by mass. Option C, the law of multiple proportions, compares the different masses of one element that combine with a fixed mass of another element in different compounds. Option D, conservation of mass, says total mass before and after a reaction remains equal. It does not specifically state gas-volume ratios. Thus, whenever the question highlights simple ratios of gaseous reactant and product volumes under the same conditions, choose the law of gaseous volumes. Exam cue: volume plus gases plus same temperature and pressure points to Gay-Lussac's law.