Explanation

(b)
$F^{-}$ is a weak field ligand and $\left\lbrack FeF_{6} \right\rbrack^{3 -}$ is an outer-orbital complex
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$\left\lbrack FeF_{6} \right\rbrack^{3 -}$ shows $sp^{3}\ d^{2}$ hybridisation and $Fe^{3 +}$ has five unpaired electrons.
In $\left\lbrack Fe(CNS)_{6} \right\rbrack^{3 -},\ CNS^{-}$ is a strong field ligand and is inner orbital complex.
$\left\lbrack Fe(CNS)_{6} \right\rbrack^{3 -}$
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$\left\lbrack Fe(CNS)_{6} \right\rbrack^{3 -}$ shows $d^{2}\ sp^{3}$ hybridisation and has one unpaired electron.
Hence, the reaction.
$\left\lbrack Fe(CNS)_{6} \right\rbrack^{3 -} \rightarrow \left\lbrack FeF_{6} \right\rbrack^{3 -}$
takes place with increase in magnetic moment.