Which isomer forms for square planar four coordinate complexes?

In square planar four-coordinate complexes, the spatial arrangement of ligands around the central metal ion can lead to geometric isomers, specifically cis and trans isomers. In the cis configuration, two identical ligands occupy adjacent positions, while in the trans configuration, they are positioned opposite each other. This distinction in the placement of the ligands results in different spatial orientations and, therefore, distinct chemical and physical properties, which is characteristic of geometric isomerism.

The other options do not apply to square planar complexes. Enantiomers refer to chiral molecules that are non-superimposable mirror images of each other, which is not the case for square planar complexes that have a symmetrical arrangement of ligands. Meso and racemic forms involve chiral and achiral molecules; however, square planar complexes can often possess a plane of symmetry, leading to meso forms rather than enantiomers. The last option mentions hexahedral and octahedral, which refers to a different geometry altogether, being typical of six-coordinate complexes rather than four-coordinate square planar complexes. Thus, the specific arrangement of ligands in square planar geometry allows for the formation of cis and trans isomers, making this answer correct.