Magnetic moment is directly proportional to the number of unpaired in the central metal atom of a complex.

It is calculated by the formula, $\mathrm{\mu }=\sqrt{\mathrm{n}(\mathrm{n}+2)}\mathrm{BM}$

(A) ${\left[{\mathrm{FeF}}_6\right]}^{3-}$

${\mathrm{Fe}}^{+3}\to 3{\mathrm{d}}^{5}4{\mathrm{s}}^0$

$\mathrm{n}=5$

(B) ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$

${\mathrm{Fe}}^{+3}\to 3{\mathrm{d}}^{5}4{\mathrm{s}}^0$

$\mathrm{n}=1$

(C) ${\left[{\mathrm{MnCl}}_6\right]}^{3-}$

${\mathrm{Mn}}^{+3}\to 3{\mathrm{d}}^{4}4{\mathrm{s}}^0$

$\mathrm{n}=4$

(D) ${\left[\mathrm{Mn}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$

${\mathrm{Mn}}^{+3}\to 3{\mathrm{d}}^{4}4{\mathrm{s}}^0$

$\mathrm{n}=2$

$\mathrm{Hence}, \mathrm{correct} \mathrm{order} \mathrm{of} \mathrm{magnetic} \mathrm{moment} \mathrm{is} \mathrm{A}>\mathrm{C}>\mathrm{D}>\mathrm{B}$