$\mathrm{Cr}^{+2} \Rightarrow[\mathrm{Ar}] 3 \mathrm{d}^{4}$

$\mathrm{H}_{2} \mathrm{O} \rightarrow$ Weak field ligand

$\text { Unpaired } \mathrm{e}^{-}=4$

$\text { Magnetic moment } =\sqrt{24} \mathrm{BM} =4.89 \mathrm{BM}$

$(II)\;\;\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}$

$\mathrm{Fe}^{+2} \Rightarrow[\mathrm{Ar}] 3 \mathrm{d}^{6}$

$\mathrm{CN}^{-} \rightarrow$ Strong field ligand

Unpaired $\mathrm{e}^{-}=0$

Magnetic moment $=0 \mathrm{BM}$ $=0 \mathrm{BM}$

$\mathrm{III}\;\;\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]^{3-}$

$\mathrm{Fe}^{+3} \Rightarrow[\mathrm{Ar}] 3 \mathrm{d}^{5}$

As $\Delta_{0}>P$

Unpaired $\mathrm{e}^{-}=1$

$\text { Magnetic moment } =\sqrt{3} \mathrm{BM}$$=1.73 \mathrm{BM}$

$(IV)\;\;\left(\mathrm{Et}_{4} \mathrm{N}\right)^{+}\left[\mathrm{CoCl}_{4}\right]^{2-}$

$\mathrm{Co}^{+2} \Rightarrow[\mathrm{Ar}] 3 \mathrm{d}^{7}$

Unpaired electrons $=3$

$\begin{aligned} \text { Magnetic moment } &=\sqrt{15} \mathrm{BM} \\ &=3.87 \mathrm{BM} \end{aligned}$

Hence order of magnetic moment is $I>IV>III>II$