Which of the following equations represents a reaction for which the delta H can be called a standard enthalpy of formation, delta Hf?

Question

a) FeO(s) + CO2(g) -> FeCO3(s)
b) 1/2 N2(g) + 2 O(g) -> NO2(g)
c) 2 Na(s) + Cl2(g) -> 2 NaCl(s)
d) Ba(s) + N2(g) + 3 O2(g) -> Ba(NO3)2(s)
e) more than one of the four equations above are standard enthalpies of formation.

I chose answer e.
is it correct?

Answer 1

No.
a is out because it contains compounds on the left.
b is out because O is not the standard state.
c is out because coefficient NaCl is 2.
e. can't be right since I've listed all but d.