1. Identify the geometry of each complex

   • Complexes of the type $[M(AA)_2X_2]$ with coordination number $6$ are typically octahedral when $AA$ is a bidentate ligand like en.
   • Complexes such as $[PtCl_2(NH_3)_2]$ are square planar for $Pt(II)$.

2. Check each option for chirality

   A. $\mathrm{cis-[PtCl_2(en)_2]^{2+}}$

   • Here, en is a bidentate ligand, so the complex is of the form $[M(en)_2Cl_2]$.
   • Such a complex is octahedral.
   • In the cis form, the arrangement lacks a plane of symmetry and exists as non-superimposable mirror images.
   • Therefore, it is chiral.

   B. $\mathrm{trans-[Co(NH_3)_4Cl_2]^{+}}$

   • This is an octahedral complex of type $[MA_4B_2]$.
   • The trans isomer has symmetry elements (plane/center of symmetry), so it is achiral.

   C. $\mathrm{trans-[PtCl_2(en)_2]^{2+}}$

   • This is again of type $[M(en)_2Cl_2]$ octahedral.
   • The trans form has symmetry and is achiral.

   D. $\mathrm{cis-[PtCl_2(NH_3)_2]}$

   • This is a square planar complex.
   • Square planar complexes lie in one plane, so the molecular plane itself is a plane of symmetry.
   • Hence it is achiral.

3. Conclusion

   Only option A is chiral.

$\boxed{\mathrm{cis-[PtCl_2(en)_2]^{2+}}}$