(i) [FeCl4]2−⟶Fe2+⟶[Ar]3d6 ∴Cl is weak field ligand so does not pairing occur
So, magnetic moment (µ)=√n(n+2) BM =√4(4+2) BM ‌‌(n= Number of total unpaired e−=4) =√24BM=4.90BM (ii) [Co(C2O4)3]3−⟶Co3+⟶[Ar]3d6 C2O4 is strong field ligand so pairing occur.
All electrons are paired, n=0 hence, µ=0 (iii) MnO42−→Mn+6→[Ar]Bd1 n=1 =√n(n+2)BM =√1(1+2)BM =√3BM =1.73BM