The phenomenon of facial and meridional isomerism occurs in octahedral complexes with a general formula of the type MA3B3, where A and B are different sets of bidentate or monodentate ligands. This type of isomerism describes the distinct arrangement positions these ligands can adopt relative to each other in the geometry of the complex. Facial (fac) isomerism occurs when the three identical ligands occupy adjacent positions at the corners of one face of the octahedron, forming a triangle. This can be thought of as the three A ligands or three B ligands clustering on the same face of the octahedral structure. Meridional (mer) isomerism, on the other hand, is characterized by the arrangement where the three identical ligands lie along a meridian roughly forming a vertical band that includes the metal center. This isomer has the three A ligands or three B ligands spread out, each occupying a vertex of a plane that cuts through the center of the octahedron and divides it into two equal halves. Examining the options provided: Option A: [Co(H2O)3(NO2)3] is an octahedral complex where we have three molecules of water and three nitro groups as ligands. This type of complex has the general formula MA3B3, and thus can exhibit both facial and meridional isomerism, making it a possible answer. Option B: [Co(‌ en ‌)2‌Cl2]+does not fit the required general formula MA3B3 because it contains two ethylenediamine (en) ligands and two chloride ions. This complex is also more suitable to show geometrical isomerism (cis/trans). Option C : [Co(NH3)4‌Br2]NO3 does not fit MA3B3 formula either, as it has four ammonia and two bromide ligands, making it suitable for other types of isomerism like cis-trans but not facial or meridional. Option D: [Co(en3]‌Cl3. does not fit the formulational criterion MA3B3; it is a complex with three identical chelate rings (triethylenediamine ligands), and symmetry precludes it from fac or mer isomerism. Conclusion: Among the options, Option A [Co(H2O)3(NO2)3] is the only complex capable of exhibiting both facial and meridional isomerism due to its meeting the MA3B3 configuration criteria. The other options do not conform to this requirement and thus cannot exhibit these specific types of isomerism.