Understanding the Concept:
In coordination chemistry, stereoisomerism is dictated by geometry:
Square planar complexes of the form $[\text{MA}_2\text{B}_2]$ exhibit geometric ($\text{cis-trans}$) isomerism but possess a plane of symmetry, making them optically inactive.
Octahedral complexes containing bidentate chelating ligands (like oxalate, $\text{ox}$) lack planes of symmetry in certain configurations, giving rise to non-superimposable mirror images (chiral optical enantiomers).
Step 1: Analyze Option (B).
The complex $[\text{CrCl}_2(\text{ox})_2]^{3-}$ is an octahedral structure of the type $[\text{MA}_2(\text{AA})_2]$:
The trans isomer places the two chloride ligands $180^\circ$ apart, creating a vertical plane of symmetry that renders it optically inactive.
The cis isomer places the chloride ligands $90^\circ$ apart. This orientation forces the two chelating oxalate loops into perpendicular planes, eliminating any internal plane of symmetry. As a result, the cis form is chiral and resolves into optically active $d$ and $l$ enantiomers.
This matches description (B) perfectly.