To evaluate the given statements, we need to consider the concepts of oxidation states and bonding tendencies across different periods in the periodic table.

Statement (I) describes the oxidation state of an element in a compound as the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic. This consideration does involve looking at how electrons are shared or transferred between atoms, but it's not solely based on electron gain enthalpy. Rather, the oxidation state is determined by a set of rules which include, but are not limited to, electronegativity differences and known oxidation states of elements. Therefore, the statement is somewhat misleading in its specificity about electron gain enthalpy being the sole consideration. The electron gain enthalpy is a measure of the energy change when an electron is added to a neutral atom in the gaseous state to form a negative ion, which indirectly influences the discussion of oxidation states but is not the direct basis for their determination.

Statement (II) points out that $$\mathrm{p}\pi-\mathrm{p}\pi$$ bond formation is more prevalent among second period elements compared to elements in other periods. This is correct and can be attributed to the smaller size of the second period elements (such as carbon, nitrogen, and oxygen), which allows their p orbitals to overlap more effectively for $$\mathrm{p}\pi-\mathrm{p}\pi$$ bonding. As we move down the periods, the atomic size increases, which leads to less effective overlapping of p orbitals for $$\mathrm{p}\pi-\mathrm{p}\pi$$ bonding due to increased distance between the valence electrons and nucleus. This makes $$\mathrm{p}\pi-\mathrm{p}\pi$$ bonds less common and less strong in elements of higher periods compared to those in the second period.

Therefore, the most accurate assessment of the statements given the explanations above is:

• Statement I is not entirely accurate because it overemphasizes electron gain enthalpy in determining oxidation states.
• Statement II is correct as it accurately describes the prevalence of $$\mathrm{p}\pi-\mathrm{p}\pi$$ bonding among second period elements.

Hence, the correct option is:

Option C: Statement I is incorrect but Statement II is correct.