Which one of the following platinum complexes has a square planar geometry?

In $\mathrm{K_2[PtCl_4]}$ platinum is in the +2 state with a $d^8$ configuration and four ligands, and $d^8$ ions of the heavier transition metals form square planar complexes. In $\mathrm{K_2[PtCl_6]}$, $\mathrm{[Pt(en)_3]Cl_4}$ and $\mathrm{[Pt(NH_3)_6]Cl_4}$ platinum is in the +4 state ($d^6$) and six-coordinate, whether from six monodentate ligands or three bidentate ethylenediamine ligands, so these complexes are octahedral.