Ionization energy, in chemistry and physics, the amount of energy required to remove an electron from an isolated atom or molecule.
3rd Ionisation energy:
V:2833kJ/mol
Cr:2990kJ/mol
Mn:3260kJ/mol
Fe:2962kJ/mol
alternative
Mn:3d54s2
Fe:3d64s2
Cr:3d54s1
V:3d34s2
So Mn has highest 3rdIE among all the given elements due to d5configuration