What is a characteristic of two compounds that can be used to demonstrate the Law of multiple proportions?

Question

They both must have carbon atoms

they both must burn in the presence of oxygen

they both must have the same mass

they both must be made of the same elements

Answer 1

If I'm reading this right, the answer is both must have the same mass.

Law 3: Law of Multiple Proportions When two elements combine to form more than one compound, the mass of element A which combines in the first compound with a given amount of element B has a simple whole number ratio with the mass of element A which combines in the second compound with the same given mass of element B. This sounds confusing, but an example clarifies this statement. Consider the carbon oxides, and let carbon be element B and oxygen be element A. Take a fixed given mass of carbon (element B), say 1 gram. The mass of oxygen which combines with 1 gram of carbon to form the first oxide is 1.33 grams. The mass of oxygen which combines with 1 gram of carbon to form the second oxide is 2.66. These masses are in ratio 2.66:1.33=2:1, a simple whole number ratio. In explaining our observations of the Law of Multiple Proportions for the carbon oxides and the nitrogen oxides, we have concluded that the simple mass ratio arises from the simple ratio of atoms contained in the individual molecules. Thus, we have established the following postulates of the Atomic Molecular Theory.

Answer 2

THEY BOTH MUST BE MADE OF THE SAME ELEMENTS.
e.g. NO2 and NO3, both are made up of Nitrogen and Oxygen.