(b)
When ligands are exchanged between metal atoms, coordination isomerism results. Hence, $\lbrack Co\left( {NH}_{3} \right)_{6}\rbrack\lbrack Cr(CN)_{6}\rbrack\ and\ \lbrack Cr\left( {NH}_{3} \right)_{6}\rbrack\lbrack Co(CN)_{6}\rbrack\$represents coordination isomerism.