The correct order of spin-only magnetic moments among the following is (Atomic number : Mn = 25, Co = 27, Ni = 28 and Zn = 30)

A complex's spin-only magnetic moment will be greater if there are more unpaired electrons in it.
In these complexes, the central metal ion is in +2 oxidation state.

$$\begin{gathered} {\mathrm{Zn}}^{2+} \mathrm{has} 3{\mathrm{d}}^{10} \mathrm{with} 0 \mathrm{unpaired} \mathrm{electrons}. \\ {\mathrm{Ni}}^{2+} \mathrm{has} 3{\mathrm{d}}^8 \mathrm{with} 2 \mathrm{unpaired} \mathrm{electrons}. \\ {\mathrm{Co}}^{2+} \mathrm{has} 3{\mathrm{d}}^7 \mathrm{with} 3 \mathrm{unpaired} \mathrm{electrons}. \\ {\mathrm{Mn}}^{2+} \mathrm{has} 3{\mathrm{d}}^5 \mathrm{with} 5 \mathrm{unpaired} \mathrm{electrons}. \end{gathered}$$
Hence, the correct option is option (D) ${\left[{\mathrm{MnCl}}_4\right]}^{2-} > {\left[{\mathrm{CoCl}}_4\right]}^{2-} > {\left[{\mathrm{NiCl}}_4\right]}^{2-} > {\left[{\mathrm{CoCl}}_4\right]}^{2-}$.