Which of the reaction defines molar  ${\mathrm{\Delta H}}_{\mathrm{f}}^{\circ }$ ?

enthalpy of formation typical When one mole of a substance is made under normal circumstances from its pure constituents, the amount of energy released or used is known as ${\mathrm{\Delta H}}_{\mathrm{f}}^{\circ }$. All of the reactants in the reaction of option C are pure elementary gases.