The electronic configuration with the highest ionisation enthalpy is

The electronic configuration with the highest ionisation enthalpy is $\left[\mathrm{Ne}\right]3{ \mathrm{s}}^{2}3{\mathrm{p}}^3$ . On moving down the group, the ionisation enthalpy decreases. In a period, on moving from left to right, the ionisation enthalpy increases.

Hence, the required option is C.