Identify the element which has highest second ionisation enthalpy.

As we move across a period, effective nuclear charge $\left(Z_{\text {eff }}\right)$ increases and so do the ionisation energy and moving down a group, effective nuclear charge $\left(Z_{\text {eff }}\right)$ decreases and so do the ionisation energy. So, phosphorus belonging to 3 rd period will have less ionisation energy than 2nd period elements. Considering the first ionisation energy of 2nd period elements, $\mathrm{N}$ is greater than $\mathrm{O}$ as $\mathrm{N}$ has half-filled stable electronic configuration, but after the removal of 1 electron, oxygen acquires a half-filled stable electronic configuration, the second ionisation potential of oxygen becomes more than of $\mathrm{N}$ and $\mathrm{C}$.