Octahedral complexes are coordination compounds that have six ligands surrounding a central metal ion. Geometric isomerism occurs when there are different possible arrangements of ligands around the metal ion. In order for geometric isomerism to occur, the complex must have at least two different types of ligands.

To determine which of the given octahedral complexes will not show geometric isomerism, we need to analyze the ligands present in each complex.

a) [MA5B]
This complex has only two types of ligands: A and B. Since there are only two ligands present, it is not possible to have different arrangements of ligands around the metal ion. Therefore, this complex will not show geometric isomerism.

b) [MA2B4]
This complex has two types of ligands: A and B. There are two A ligands and four B ligands. The A ligands can occupy two different positions, while the B ligands can occupy four different positions. Therefore, this complex can exhibit geometric isomerism.

c) [MA3B3]
This complex also has two types of ligands: A and B. There are three A ligands and three B ligands. The A ligands can occupy three different positions, while the B ligands can occupy three different positions. Therefore, this complex can exhibit geometric isomerism.

d) [MA4B2]
This complex has two types of ligands: A and B. There are four A ligands and two B ligands. The A ligands can occupy four different positions, while the B ligands can occupy two different positions. Therefore, this complex can exhibit geometric isomerism.

In summary, the octahedral complex that will not show geometric isomerism is [MA5B] because it has only two types of ligands, which does not allow for different arrangements around the metal ion.