$(i)$ $\left[ FeF _{6}\right]^{3-} Fe ^{3+} \Rightarrow[ Ar ] 3 d ^{5}, F ^{-}$ is $WFL$

$n =5, \quad \mu=\sqrt{35}\, B .M.$

$(ii)$ $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+} \Rightarrow Co ^{3+}[ Ar ] 3 d ^{6}, NH _{3}$ is $SFL$

$n =0, \quad \mu=0$

$(iii)$ $\left[ NiCl _{4}\right]^{2-} Ni ^{2+} \Rightarrow[ Ar ] 3 d ^{8}, Cl ^{-} \rightarrow WFL$

$n =2, \quad \mu=\sqrt{8}\, B . M.$

$(iv)$ $\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+} Cu ^{2+} \Rightarrow[ Ar ] 3 d ^{9}, NH _{3} \rightarrow \,SFL$

$n =1, \quad \mu=\sqrt{3} B.M.$

Thus correct order of spin only magnetic moment is $( i ) > ( iii ) > ( iv ) > ( ii )$