Geometrical isomerism is a type of stereoisomerism that arises due to the restricted rotation around a bond in a molecule. It occurs when two different groups or atoms are attached to the central metal atom or ion in a coordination complex, resulting in different spatial arrangements of the ligands. In other words, geometrical isomerism occurs when there is a possibility of different arrangements of ligands around the central metal atom.

In the given options, the complexes that show geometrical isomerism are octahedral (option B) and square planar (option C). Let's understand why these complexes exhibit geometrical isomerism.

b) Octahedral Complexes:
In octahedral complexes, the central metal atom is surrounded by six ligands arranged in an octahedral geometry. The ligands can be arranged in two different ways to form geometrical isomers - cis and trans isomers.

- Cis isomer: In the cis isomer, two identical ligands are adjacent to each other in the same plane.
- Trans isomer: In the trans isomer, the two identical ligands are placed opposite to each other in different planes.

For example, in the complex [Co(NH3)4Cl2]+, there can be two possible arrangements of ligands:
- Cis isomer: [Co(NH3)4Cl2]+ with two chloride ligands adjacent to each other in the same plane.
- Trans isomer: [Co(NH3)4Cl2]+ with two chloride ligands placed opposite to each other in different planes.

c) Square Planar Complexes:
In square planar complexes, the central metal atom is surrounded by four ligands arranged in a square planar geometry. The ligands can be arranged in two different ways to form geometrical isomers - cis and trans isomers.

- Cis isomer: In the cis isomer, two identical ligands are adjacent to each other in the same plane.
- Trans isomer: In the trans isomer, the two identical ligands are placed opposite to each other in different planes.

For example, in the complex [Pt(NH3)2Cl2], there can be two possible arrangements of ligands:
- Cis isomer: [Pt(NH3)2Cl2] with two chloride ligands adjacent to each other in the same plane.
- Trans isomer: [Pt(NH3)2Cl2] with two chloride ligands placed opposite to each other in different planes.

Therefore, octahedral and square planar complexes show geometrical isomerism due to the different arrangements of ligands around the central metal atom.