Which of the following complex is optically inactive but its other geometrical isomer is optically active?

Geometrical isomerism is observed in the complexes $\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_2\right]$ and $\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_4\right]$. They come in both cis and trans isomers. They do not, however, exhibit optical isomerism.

${\left[\mathrm{Pt}(\mathrm{en}{)}_3\right]}^{4+}$ exhibits optical isomerism. It comes in two isomers: d and l. It exhibits no geometrical isomerism.

There are two geometrical isomers of the complex $\left[\mathrm{Pt}(\text{ en }{)}_2{\mathrm{Cl}}_2\right]$ cis and trans. The optically active cis isomer exists in d and l forms. The trans isomer has no optical activity.

$\text{ cis-[Co(en }\left.{ }_2{\left({\mathrm{NH}}_3\right)}_2\right]{\mathrm{Cl}}_3$ is optically active.

So, option C is correct answer.