The law of multiple proportions states that ‘if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.’

The first option does not follow the law because it is in regards to only carbon dioxide as a product of carbon and oxygen.

In the second option, the mass of oxygen in CO₂ is 16x2 = 32 and the mass in CO is 16x1 = 16. The ratio of the masses of oxygen is in the ratio 32:16 that is 2:1. Hence it follows the law of multiple proportions.

In the third option, the reaction of burning magnesium to magnesium oxide is given by 2Mg + O₂→ 2MgO. The amount of magnesium taken for the reaction is not equal to the amount of magnesium present in the molecule, hence it does not follow law of multiple proportions.

In the fourth option, the formation of water vapour does not follow law of multiple proportions but Gay-Lussac’s law which states ‘when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure.’

The correct option is →ii).