Explanation

[Solution from source] The second electron is removed from M⁺. Cr⁺ is 3d⁵, a stable half-filled shell, so removing a 3d electron costs the most. Mn⁺ (3d⁵4s¹) loses its 4s electron more easily, and V⁺ and Ti⁺ follow in order of nuclear charge. Observed values (≈1590, 1510, 1410, 1310 kJ mol⁻¹) give Cr > Mn > V > Ti. Mn > Cr would hold for the third, not the second, ionisation enthalpy. Answer (2).