What is the value of spin only magnetic moment for $Mn^{2+}$ in $BM$?

What is the reason for the deviation between the experimental and theoretical values of magnetic moment?

Assertion : $[Fe(CN)_6]^{3-}$ is weakly paramagnetic while $[Fe(CN)_6]^{4-}$ is diamagnetic.
Reason : $[Fe(CN)_6]^{3-}$ has $+3$ oxidation state while $[Fe(CN)_6]^{4-}$ has $+2$ oxidation state.

Among the following complexes,the one that can exist as facial $(fac)$ and meridional $(mer)$ isomers is

The magnetic moment of the complex $[Cr(CN)_6]^{3-}$ is ..... (in $BM$)

Identify the ion (hydrated in solution) which is not correctly matched with its spin only magnetic moment (in $BM$) given in brackets.