The one that is not expected to show isomerism is:

(1) $\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_2\right]$ does not show isomerism because it is tetrahedral and has a plane of symmetry.

(2)${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_4{\left({\mathrm{H}}_2\mathrm{O}\right)}_2\right]}^{2+}$shows geometrical isomerism.

(3)${\left[\mathrm{Ni}(\mathrm{en}{)}_3\right]}^{2+}$ , shows optical isomerism.
(4)$\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{C}}_2\right],$ shows geometrical isomerism.