2. The standard enthalpy of formation of N2O4 (g) at 25 C is 9.16 kJ/mol. To which of the following chemical reactions does this enthalpy change apply?
A. 2N (g) + 4O (g) --> N2O4 (g)
B. N2 (g) + 2O2 (g) --> N2O4 (g)
C. 2 NO2 (g) --> N2O4 (g)
D. N2O4 (l) --> N2O4 (g)

The correct answer is B. How would I be able to answer questions like this? I thought it might relate to the number of moles of gas on each side of the equation, but thats the same for both A and B.  Is it because '4O' and 2N would not naturally be found since O and N are diatomic, leaving B as the only valid answer?

enthalpy of formation is defined as the formation of a compound from its element .
Now N2O4 is made up of N and O
so its elements are
N2 and O2
That's the reason B is correct .In other option all are compounds and A is not correct because elemental form in which nitrogen can exist is N2 and not N .