If 1 volume chlorine and 1 volume hydrogen form 2 volumes hydrogen chloride which law applies?

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

The direct answer is option A: Gay-Lussac's law of gaseous volumes. The statement gives volumes of gases: 1 volume of chlorine, 1 volume of hydrogen, and 2 volumes of hydrogen chloride. These are in the simple ratio 1:1:2, under the usual condition that the gas volumes are measured at the same temperature and pressure. Step 1: identify that the data are volumes, not masses. Step 2: note that all substances involved are gases in this gaseous-volume example. Step 3: write the ratio as 1:1:2. Step 4: match a simple gas-volume ratio with Gay-Lussac's law. Option A is therefore correct. Option B concerns fixed mass composition in one compound, not gas-volume ratios. Option C compares mass ratios in different compounds made by the same elements, so it is not relevant. Option D, reciprocal proportions, deals with mass relationships among elements and compounds, not the stated gas volumes. The important exam cue is simple volume ratios in a gas reaction; that points directly to Gay-Lussac's law.