Correct Answer - ABC
(A) Greater the number of valence electrons, stronger is the resultant bonding and thus greater is the enthalpies of atomization.
(B) `Cr^(+)` has half-filled valence shell electron configuration `3d^(5)`, so it has higher second ionisation energy. Similarly `Cu^(+)` has completely filled stable valence shell electron configuration `3d^(10)`, So it has higher second ionisation energy.
(C) Sum of first two ionisation energies `(IE_(1)+IE_(2))` of `Ni(II)` is less than that of `Pt(II)`. Similarly sum of first four ionisation energies of `Pt(IV)` is less than that of `Ni(IV)`.
(D) Element occurs in the middle of the period, e.g., manganese has `+7` oxidation state as it contain five unpaired electrons.