For which of the following reactions does ΔHrxn represent an enthalpy of formation?

a 2N₂(g) + 3O₂(g) => 2NO₂(g) + 2NO(g)
b N₂(g) + O₂(g) => 2NO(g)
c 2NO₂(g) => N₂O₂(g)
d N₂(g) + 2O₂(g) => 2NO₂(g)