a
When two complexes have same number of unpaired electrons, they have same value of spin only magnetic moment.

The number of unpaired electrons present in

\(\left[ Cr \left( H _2 O \right)_6\right]^{3+},\left[ Mn \left( H _2 O \right)_6\right]^{4+},\left[ Mn \left( H _2 O \right)_6\right]^{3+}\)

\(,\left[ Fe \left( H _2 O \right)_6\right]^{3+} \text { and }\left[ Cu \left( H _3 N \right)_4\right]^{2+} \text { are }\)

\(3,3,4,5\) and \(1\) respectively.

Thus the magnetic moment of \(\left[ Mn \left( H _2 O \right)_6\right]^{4+}\) is same as that of \(\left[ Cr \left( H _2 O \right)_6\right]^{3+}\).