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 $\left[ Cr \left( H _2 O \right)_6\right]^{3+}$.