A report says in (CO) and (CO_2) a fixed mass of carbon combines with different masses of oxygen. Which law applies?

Correct answer: Multiple proportions

The direct answer is option B: the law of multiple proportions. The concept is used when the same two elements form more than one compound, and a fixed mass of one element combines with different masses of the other element in a simple whole-number ratio. CO and CO₂ contain the same two elements, carbon and oxygen, but in different compounds. To reason step by step, fix the mass of carbon, compare the oxygen masses combined with it in CO and CO₂, and reduce their ratio; the ratio is a simple whole-number ratio. Option A, conservation of mass, concerns equality of total mass before and after a reaction, not comparison between two compounds. Option B is correct because it compares different compounds made from the same two elements. Option C, definite proportions, applies to one pure compound and says its elements have a fixed mass ratio; it does not explain the comparison between CO and CO₂. Option D, gaseous volumes, concerns volume ratios of gases at the same temperature and pressure, not these mass combinations. Memory cue: same two elements, different compounds, different masses in a simple ratio means multiple proportions.