Which pair of reaction is an example for law of multiple proportions ?

Law of multiple proportion = when two elements combine to form more than one product, then masses of one element that combine with fixed mass of another element, will always be a simple whole number ratio.According to the above statement reactants in both the reactions must be same; therefore option ‘b’ is correct.In option 1, 3 and 4 reactants in both reactions are different, therefore they don’t obey law of multiple proportions.