Which of the reaction defines ΔH0f ?
ΔH0f is the enthalpy change when 1 mole of the substance is formed from its elements in their standard states.
In (a) carbon is present in diamond however standard state of carbon is graphite. Again, in (d) CO (g) is involved so it can't be the right option. Further in (c) 2 moles of NH3 are generated. Hence the correct option is (b)