Among the following, third ionisation energy is highest for :

The electronic configuration of the given elements is as

$\begin{array}{r}{ }_{12}\mathrm{Mg}=\left[\mathrm{Ne}\right]3{\mathrm{s}}^2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{ }_5\mathrm{B}=\left[\mathrm{He}\right]2{\mathrm{s}}^2,2{\mathrm{p}}^1\\ { }_4\mathrm{Be}=\left[\mathrm{He}\right]2{\mathrm{s}}^2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{ }_{13}\mathrm{Al}=\left[\mathrm{Ne}\right]3{\mathrm{s}}^2,3{\mathrm{p}}^1\end{array}$

From the electronic configuration, it is clear that third ionisation energy of Mg and Be is higher as compared to AI and B as in them, the third electron have to be removed from the stable inert gas configuration.
Between Mg and Be, third IE of Be is high because of its smaller size than Mg as both lie in the same group.