Final answer:
The standard enthalpy of formation for an element in its most stable form is zero by definition, because no formation process is needed for an element in its native state. Therefore, the answer to the question is b) Zero.
Step-by-step explanation:
The standard enthalpy of formation (denoted as ΔHf or ΔHformation) for an element in its most stable form is zero. This is because the standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance is formed from its constituent elements in their standard states. The standard state of an element is its most stable form at 1 atm pressure and 25°C.
For example, for oxygen, the most stable form is O₂ (g), for hydrogen, it is H₂ (g), and for carbon, it is graphite. These forms have ΔH values of zero since they are in their elemental state and no formation is actually occurring. Therefore, the answer to the question is b) Zero.