Which statement best describes the standard enthalpy of formation, ${\mathrm{\Delta H}}_f\circ$, of a substance?

• A. The enthalpy change for forming $1$ mol of the substance from any reactants at equilibrium under standard conditions
• B. The enthalpy change for decomposing $1$ mol of the substance into its elements in their gaseous states at any pressure
• C. The enthalpy change for forming $1$ mol of the substance from its constituent elements in their standard states at specified conditions (typically $1$ bar and $298$ K)
• D. The enthalpy change for burning $1$ mol of the substance completely in $O_2$ under standard conditions

Answer: (C)