The empirical formula of a compound is the formula with the lowest and simplest possible whole number ratio of elements, this means that we can transform molecular formulas with higher numbers into lower, more compact numbers, one example is:
N2O4 turning into NO2
We did that by dividing all the whole numbers by 2
C5H11 = C10H22, again the empirical formula was the result of a division by 2
In our question we have C2H4, which is not an empirical formula, since we make it even simpler, turning it into CH2
C4H10 is not an empirical formula because we can make it even simpler by turning it into C2H5
P2O5 is an empirical formula, because we can't make it simpler
P4O6 can't be an empirical formula, because we can turn it into P2O3
Therefore the correct one is letter C