In which law are volumes of gaseous substances taken under the same conditions?

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

Gay-Lussac’s law of gaseous volumes compares the volumes of reacting gases, or gaseous products, when temperature and pressure are kept the same. Under these common conditions, the volumes show simple whole-number relationships. The phrase “same conditions” is the important clue in the question. It prevents an unfair comparison in which temperature or pressure changes the volume independently of the chemical reaction.

Thus option A is correct. The law of definite proportions concerns the fixed mass composition of a compound, and the law of multiple proportions compares mass ratios in different compounds formed by the same elements. Neither is specifically a law about reacting gas volumes under equal conditions. The supplied explanation is accurate, although the more complete statement is that both temperature and pressure must be constant, not merely generally similar.