In ionic compounds, the compound is a lattice of the elemnts in it that repeats over and over in the same ratios, so for ionc compounds you can simplify the ratios.
On the other hand, in molecules, each molecule is unic and independe from the others, so it is not a repeating lattice, each molecule has its unique amounts of each element. So, in molecules, you can't reduce the ratios of the elements on it.
In N₂O₄, we have two non metals, N and O, so they are bonded by covalent bonds. Thus, N₂O₄ is a molecules, and being so we can't reduce the ratios of its atoms.
Thus, the correct alternative is alternative B.
