Among Ti, V, Mn and Fe, the metal with the highest third ionisation enthalpy is

The third ionisation removes an electron from $\mathrm{M}{}^{2+}$. $\mathrm{Mn}{}^{2+}$ is $3d^5$ (half-filled), so removing the next electron is hardest, whereas $\mathrm{Fe}{}^{2+}$ ($d^6$) gains stability on becoming $d^5$. Ti and V are lower still, so Mn is highest.