Chemistry - Complete d-block Question with Solution | TestHub

The colour of transition metal ions is generally due to the presence of unpaired d-electrons which can undergo d-d transitions.

The electronic configurations of the given ions are:
${\mathrm{Zn}}^{2+}:[\mathrm{Ar}]3d^{10}$ (0 unpaired electrons)
${\mathrm{Fe}}^{2+}:[\mathrm{Ar}]3d^6$ (4 unpaired electrons)
${\mathrm{Cu}}^{+}:[\mathrm{Ar}]3d^{10}$ (0 unpaired electrons)
${\mathrm{Sc}}^{3+}:[\mathrm{Ar}]3d^0$ (0 unpaired electrons)

Since only ${\mathrm{Fe}}^{2+}$ has unpaired electrons in its d-orbital, it forms coloured compounds.

Answer: ${\mathrm{Fe}}^{2+}$