Which one of the following complexes can exhibit geometrical isomerism?

Question

$\begin{array}{1 1}(a)\;[Pt(NH_3)2Cl_2],(square planar)\\(b)\;[Zn(NH_3)_2Cl_2],(tetrahedral)\\(c)\;[Cu(NH_3)_4]^{2+},(square planar)\\(d)\;[Co(NH_3)_5Cl]^{2+},(octahedral) \end{array}$