Explanation

(b)
$K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack 4K^{+} + \left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$
The oxidation number of Fe in $\left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$ is +2.
$Fe^{2 +}$ : $1s^{2},2s^{2}2p^{6},\ 3s^{2}3p^{6}3d^{6},\ 4s^{0}4p$
3$d^{6}$ 4$s$ 4$p$
[table] ⥮ | ⥮ | ⥮ | $\times$ | $\times$ | | $\times$ | | $\times$ | $\times$ | $\times$ [/table]
CN CN CN CN CN CN
Since, $CN^{-}$is a strong field ligand, pairing occurs and the hybridisation of $\left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$ is $d^{2}sp^{3}$ and structure is octahedral.