Step 1: Understanding Dalton’s Contribution
John Dalton proposed the Law of Multiple Proportions, which states that when two elements combine to form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element are small whole numbers.
Step 2: Why Other Options are Incorrect
Option (b) is incorrect because Avogadro's Law was proposed by Amedeo Avogadro and relates to the volume of gases at the same temperature and pressure, not Dalton.
Option (c) is incorrect because the Law of Definite Composition was also proposed by Dalton, but the law being asked about in this question is the Law of Multiple Proportions.
Option (d) is incorrect because the Law of Conservation of Mass was proposed by Antoine Lavoisier, not Dalton.
Step 3: Conclusion
Thus, Dalton proposed the Law of Multiple Proportions.