\(\Delta H^\circ\) represents the standard enthalpy of formation. For transition metals, this value is influenced by electron configuration, elemental stability, and ionization energy.
- Scandium (Sc) and chromium (Cr) exhibit high ionization energies and enthalpy changes.
- Copper (Cu) possesses a stable electron configuration due to a full d-subshell, resulting in lower reactivity and a moderate \(\Delta H^\circ\).
- Zinc (Zn) also has a full 3d subshell and a stable electron configuration, making it the least reactive of the 3d transition elements and consequently exhibiting the lowest enthalpy change. Therefore, Zn has the lowest \(\Delta H^\circ\).
Consequently, Zn is the correct answer.