The correct option is B 4.90,0 and 1.73 BM
The spin only magnetic moment formula-
μ=√n(n+2)B.M
n = no. of unpaired electrons.
[FeCl4]2−
Oxidation state of Fe in complex is +2, Fe2+
Cl is weak ligand, its configuration is (e)3 (t2)3
Thus, it has 4 unpaired electrons,
μ=4.9 BM
[Co(C2O4)3]3−→Co3+ and strong field.
Thus, its configuration is (t2g)6
No unpaired electrons, n=0
μ=0 BM
MnO2−4→Mn+6
Its configuration is [Ar]3d1
Thus, it has one unpaired electron, n=1
μ=1.73 BM
(n is number of unpaired electron and μ is spin only magnetic moment)