In octahedral complex the magnitude of $${\Delta _o}$$ will be highest in a complex having strongest ligand. Of the given ligands $$C{N^ - }$$ is strongest so $${\Delta _o}$$ will be highest for $${\left( {Co{{\left( {CN} \right)}_6}} \right)^{3 - }}.$$ Thus option $$(a)$$ is correct.