Which of the following complexes exhibits optical activity?

The pair having the same magnetic moment is $:$
[At. No.$: $ $Cr= 24, Mn= 25, Fe= 26, Co= 27$]

Arrange the following in the increasing order of the number of unpaired electrons present in the central metal ion:
$I.$ $[MnCl_6]^{3-}$
$II.$ $[FeF_6]^{3-}$
$III.$ $[Mn(CN)_6]^{3-}$
$IV.$ $[Fe(CN)_6]^{3-}$

$[Fe(H_2O)_6]^{2+}$ and $[Fe(CN)_6]^{4-}$ differ in

Which of the following complex ions is expected to absorb light in the visible region?

Compounds with spin-only magnetic moment equivalent to five unpaired electrons are