Octahedral complexes of copper (II) undergo structural distortion (Jahn-Teller). Which one of the given copper (II) complexes will show the maximum structural distortion?
a) [Cu(en)₄]²⁺
b) [Cu(NH₃)₄]²⁺
c) [Cu(H₂O)₄]²⁺
d) [Cu(Cl)₄]²⁻