[tex]H_2O[/tex](s) has ΔHf defined as 0 kJ/mol. Hence, option A is correct.
The standard enthalpy of formation (ΔHf) or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their most stable state at standard conditions (25°C and 1atm).
Every free element in its standard state has a value of ΔHf defined as 0 kJ/mol
Thus, the given options only [tex]F_2[/tex] are present in their standard state.
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