Law of multiple proportions is not applicable for the oxide(s) of (a) carbon (b) iron (c) nitrogen (d) aluminium

According to the Law of Multiple Proportions, when two elements 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.

Now we will analyze the oxide compositions of each element given in the options. (a) Carbon Carbon forms two oxides: carbon monoxide (CO) and carbon dioxide (CO\(_2\)). The ratio of oxygen masses combining with a fixed mass of carbon in these two compounds is 1:2, which is a small whole number ratio. (b) Iron Iron forms two oxides: ferrous oxide (FeO) and ferric oxide (Fe\(_2\)O\(_3\)). For ferrous oxide, with a fixed mass of iron (1 mole), 1 mole of oxygen combines. In ferric oxide with fixed mass of iron (2 moles), 3 moles of oxygen combines. The ratio of oxygen masses combining with the fixed mass of iron in these two compounds is 3:4, which is a small whole number ratio. (c) Nitrogen Nitrogen forms several oxides: nitrous oxide (N\(_2\)O), nitric oxide (NO), nitrogen dioxide (NO\(_2\)), and nitrogen pentoxide (N\(_2\)O\(_5\)). The ratios of oxygen masses combining with a fixed mass of nitrogen in these compounds are not in small whole number ratios. (d) Aluminium Aluminium forms only one oxide: aluminium oxide (Al\(_2\)O\(_3\)). Since there is only one compound, the Law of Multiple Proportions does not apply.

From the analysis above, we can see that the Law of Multiple Proportions is not applicable for the oxide(s) of nitrogen (option (c)) as stated in Step 2.