The correct order of magnetic moment (spin values in B.M.) is

(Atomic no. Mn = 25, Fe = 26, Co= 27)

Magnetic moment of [MnC1₄​]²⁻ is 5.9 BM.

Mn is in +2 oxidation state having configuration of [Ar]3d5. It has 5 unpaired electrons.

μ=n(n+2)​

      =(5(5+2)​

     =5.9BM

Magnetic moment of [CoC1₄​]²⁻ is 3.9 BM.

Co is in +2 oxidation state having configuration of [Ar]3d⁷. It has 3 unpaired electrons.

μ=n(n+2)​

 =(3(3+2)​

   =3.9BM

Magnetic moment of [FeCN₄]²⁻ is 0.0 BM.

Fe is in +2 oxidation state having configuration of [Ar]3d⁶. In this complex CN⁻ ion is a strong ligand and all electrons are paired and there are no unpaired electrons.

μ=n(n+2)​

 =0(0+2)​

 =0BM

The decreasing order of the magnetic moments is:

[MnCl₄​]⁻²>[CoCl₄​]²⁻>Fe(CN)₆​]⁴⁻