Step 1 - What is an empirical formula?
An empirical formula is, by defnition, a formula showing only the smallest possible proportion between the elements that are part of a substance.
Let's take glucose, C6H12O6, as an example. Note that all numbers (6, 12 and 6 again) are divisible by 6. Therefore, this is not an empirical formula. If we divide all of them by 6, we obtain CH2O, which is indeed the empirical formula for glucose.
Step 2 - How to discover whether a given formula is empirical?
It's actually quite simple: are all the small numbers multiple of some number? If yes, then it is not an empirical formula.
Step 3 - Determining which formula in the exercise is not an empirical one
Note the only formula in which the small numbers can be further divided is N2O4. We can divide them by two, which would result in NO2.
Therefore, N2O4 is not an empirical formula. The correct answer is thus item A) N2O4.