The atomic numbers of vanadium M), chromium (Cr), manganese (Mn) and iron (Fe) are respectively 23,24,25 and 26. Which one of these may be expected to have the highest second ionisation enthalpy

V(Z=23)=[Ar]3d3,4s2Cr(Z=24)=[Ar]3d5,4s1Mn(Z=25)=[Ar]3d5,4s2Fe(Z=26)[Ar]3d6,4s2Since, Cr has sable  configuration of its Cr* (3ds exactly half-filled), thus it has the highest second ionisation enthalpy.