Which of the following complexes formed by Cu2+ ions is most stable? (a) Cu2++4NH3→[Cu(NH3)4]2+, logK=11.6 (b) Cu2++4CN−→[Cu(CN)4]2−, logK=27.3 (c) Cu2++2en−→[Cu(en)2]2+, logK=15.4 (d) Cu2++4H2O→[Cu(H2O)4]2+, logK=8.9