The transition metal having highest 3 ^ rd ionisation enthalpy is:

Ionization energy, in chemistry and physics, the amount of energy required to remove an electron from an isolated atom or molecule. 

${3}^{rd}$ Ionisation energy:

$V: 2833kJ / mol$

$Cr: 2990kJ / mol$

$Mn: 3260kJ / mol$

$Fe: 2962kJ / mol$

alternative

$Mn: 3{d}^{5}4{s}^{2}$

$Fe: 3{d}^{6}4{s}^{2}$

$Cr: 3{d}^{5}4{s}^{1}$

$V: 3{d}^{3}4{s}^{2}$

So $Mn$ has highest ${3}^{rd}IE$ among all the given elements due to ${d}^{5}$configuration