Final answer:
The substance with the standard heat of formation of zero is O2(g) at 25.0 °C and 1 atm, since it is in its most stable form at standard temperature and pressure.
Step-by-step explanation:
The substance with the standard heat of formation (AH°) of zero is O2(g) at 25.0 °C and 1 atm. By definition, the standard enthalpy of formation of any element in its standard state, which is usually its most stable form at 1 atm pressure and a specified temperature (usually taken as 25°C), is zero. Therefore, option (a) O2(g) at 25.0 °C and 1 atm, which represents diatomic oxygen in its gaseous form at standard temperature and pressure, has a standard heat of formation of zero whereas the other substances do not meet the conditions for a standard heat of formation of zero because they either are not in their standard state, do not have a pressure of 1 atm, or are not at the standard temperature of 25°C.