$(i)$ $\left[ FeF _{6}\right]^{3-}$
$(ii)$ $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$
$(iii)$ $\left[ NiCl _{4}\right]^{2-}$
$(iv)$ $\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+}$
\((i)\) \(\left[ FeF _{6}\right]^{3-} Fe ^{3+} \Rightarrow[ Ar ] 3 d ^{5}, F ^{-}\) 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 \(( i ) > ( iii ) > ( iv ) > ( ii )\)