Which of the following complexes formed by Cu⁺² ion is most stable

• A

  ${\text{Cu}}^{+2}+4{\text{NH}}_3\rightleftharpoons {\left[\text{Cu}{\left({\text{NH}}_{\text{3}}\right)}_{\text{4}}\right]}^{\text{2}+};\log \text{K}=11.6$

• B

  ${\text{Cu}}^{+2}+4{\text{CN}}^{-}\rightleftharpoons {\left[\text{Cu}{\left(\text{CN}\right)}_{\text{4}}\right]}^{2-};\log \text{K}=27.3$

• C

  ${\text{Cu}}^{+2}+2\text{en}\rightleftharpoons {\left[\text{Cu}{\left(\text{en}\right)}_{\text{2}}\right]}^{2+};\log \text{K}=15.4$

• D

  ${\text{Cu}}^{+2}+4{\text{H}}_{\text{2}}\text{O}\rightleftharpoons {\left[\text{Cu}{\left({\text{H}}_{\text{2}}\text{O}\right)}_{\text{4}}\right]}^{2+};\log \text{K}=8.9$