(b)
The number of ligands attached to the central metal ion is called the coordination number.
So, coordination numbers of Fe in
$\left\lbrack Fe\ (CN)_{6} \right\rbrack^{4 -},\ \left\lbrack Fe(CN)_{6} \right\rbrack^{3 -}\ and\ \left\lbrack {FeCl}_{4} \right\rbrack^{-}\$are 6, 6 and 4 respectively.