Which statement about standard enthalpy of formation is true?

• A. The standard enthalpy of formation of a compound is always negative.
• B. The standard enthalpy of formation is measured for forming $1$ mol of a substance from its elements at $0$ $°C$ and $2$ atm.
• C. The standard enthalpy of formation equals the enthalpy change for decomposing $1$ mol of a compound into its elements in their standard states.
• D. The standard enthalpy of formation of an element in its standard state is $0$ $kJ/mol$.

Answer: (D)