$\left(\mathrm{A}\right)\mathrm{ }{\left[\mathrm{F}\mathrm{e}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+},\mathrm{ }\mathrm{F}{\mathrm{e}}^{2+}\to 3{\mathrm{d}}^6\to 4$ unpaired electron

${\left[\mathrm{C}\mathrm{r}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+},\mathrm{ }\mathrm{C}{\mathrm{r}}^{2+}\to 3{\mathrm{d}}^4\to 4$ unpaired electron

$\left(\mathrm{B}\right)\mathrm{ }{\left[\mathrm{N}\mathrm{i}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4{\left({\mathrm{H}}_2\mathrm{O}\right)}_2\right]}^{2+}=\mathrm{N}{\mathrm{i}}^{2+}\to 3{\mathrm{d}}^8$

$\to 2$ unpaired electron

${\mathrm{\mu }}_{\mathrm{m}}=2.83\mathrm{B}.\mathrm{M}$

$\left(\mathrm{C}\right)$ In gemstone, ruby has $\mathrm{C}{\mathrm{r}}^{3+}$ ion occupying the octahedral sites of aluminium oxide $\left(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)$ normally occupied by $\mathrm{A}{\mathrm{l}}^{3+}$ ion.

$\left(\mathrm{D}\right)$ Complimentary color of violet is yellow.