Correct Answer - B
`V (Z = 23) implies 3d^(3) 4s^(2)` , `Mn(Z = 25) implies 3d^(5) 4s^(2)`
`Cr(Z = 24) implies 3d^(5) 4s^(1)` , `Fe(Z = 26) implies 3d^(6) 4s^(2)`
Third ionisation enthalpy will be highest for Mn on account of extra stability of half-filled d subshell