Final answer:
The pair of compounds that can be used to illustrate the law of multiple proportions is NO and NO2.
Step-by-step explanation:
The law of multiple proportions states that when two elements combine to form more than one compound, the mass ratios of the elements in each compound will be a ratio of small whole numbers. In other words, the masses of one element that combine with a fixed mass of another element are always in small whole number ratios. Given this definition, the pair of compounds that can be used to illustrate the law of multiple proportions is option c) NO and NO2.