The standard enthalpy of formation, Delta H standard formation for nitrogen (IV) oxide, No2?

Question

is the enthalpy change for which reaction

Answer is 1/2 N2 (g) + O2 (gas) -> NO2 (g)

a) N (g) + 2O (g) -> NO2 (g)

b) above answer

c) 1/2 N2O4(g) -> No2 (g)

d) NO(g) + 1/2 O2(g) -> NO2(g)

Please explain why others can't be correct answer

Answer 1

We know the skeletal equation must be

N2 + O2 --> NO2

This means A is wrong right away.

Not sure what B means.

C is false according to the skeletal formula.

D is correct because it is the balanced equation.

[Answer: see above]