Which one of the following complexes is an outer orbital complex

Question

$\begin{array}{1 1}(a)\;[Fe(CN)_6]^{4-}&(b)\;[Ni(NH_3)_6]^{2-}\\(c)\;[CO(NH_3)_6]^{2-}&(d)\;[Mn(CN)_6]^{4-}\end{array}$