${\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+}}$

$\mathrm{Cr}^{3+}: 3 \mathrm{~d}^3$

$\mathrm{n}=3 \text { (unpaired electrons) }$

$\mu \simeq 3.87 \text { B.M. (II) }$

$(B)$

${\left[\mathrm{NiCl}_4\right]^{2-}}$

$\mathrm{Ni}^{2+}: 3 \mathrm{~d}^8$

$\mathrm{n}=2$

$\mu \simeq 2.83 \text { B.M. (IV) }$

$(C)$

${\left[\mathrm{CoF}_6\right]^{3-}}$

$\mathrm{Co}^{3+}: 3 \mathrm{~d}^6$

$\mathrm{n}=4$

$\mu \simeq 4.90 \text { B.M. (I) }$

$(D)$

${\left[\mathrm{Ni}(\mathrm{CN})_4\right]^{2-}}$

$\mathrm{Ni}^{2+}: 3 \mathrm{~d}^8$

$\mathrm{n}=0$

$\mu=0 \text { B.M. (III) }$