Which of the following complex has a possibility to exist as meridional isomer?

A meridional (mer) isomer is possible in octahedral complexes of the type $$[Ma_3b_3]$$, where three identical ligands occupy positions that include one on the axis and two in the equatorial plane (forming a T-shape or meridian).

Analyzing each option:

$$[Co(NH_3)_3(NO_2)_3]$$: This is an octahedral complex of the type $$[Ma_3b_3]$$ with three NH$$_3$$ and three NO$$_2^-$$ ligands. This can exist as both facial (fac) and meridional (mer) isomers.

$$[Pt(NH_3)_2Cl_2]$$: Square planar complex — shows cis/trans isomerism, not fac/mer.

$$[Co(en)_2Cl_2]$$: Contains bidentate en ligands — shows cis/trans isomerism.

$$[Co(en)_3]$$: Contains only one type of bidentate ligand — shows optical isomerism (D/L), not fac/mer.

Therefore, $$[Co(NH_3)_3(NO_2)_3]$$ can exist as a meridional isomer.