Complexes can exhibit geometrical isomerism when they have different spatial arrangements due to the presence of different ligands or the presence of cis-trans isomers. Geometrical isomerism arises when there is a rigid structure in the complex that prevents the interchange of ligands.

In this case, the complex that will show geometrical isomerism is (C) Aquachlorobis(ethylenediamine)cobalt(II) chloride.

Explanation:

Aquachlorobis(ethylenediamine)cobalt(II) chloride has the formula [Co(en)2Cl(H2O)]Cl. Let's break down the complex to understand its structure and identify the possible isomers.

1. Ligands:
- Ethylenediamine (en): This ligand has two nitrogen atoms that can bond with the central cobalt atom, forming a chelate ring.
- Chloride ion (Cl-): This ligand can also bond with the central cobalt atom.

2. Complex Structure:
- The central cobalt atom is bonded to two ethylenediamine ligands, forming a chelate ring.
- The central cobalt atom is also bonded to one chloride ion and one water molecule.

Now, let's consider the possible isomers of this complex:

1. Cis isomer:
- In the cis isomer, the two ethylenediamine ligands are located on the same side of the complex, while the chloride ion and water molecule are located on the opposite side.
- The cis isomer can be represented as [Co(en)2Cl(H2O)]Cl.

2. Trans isomer:
- In the trans isomer, the two ethylenediamine ligands are located on opposite sides of the complex, while the chloride ion and water molecule are located on the same side.
- The trans isomer can be represented as [Co(en)2(H2O)Cl]Cl.

These isomers have different spatial arrangements, and they cannot be interconverted without breaking and reforming bonds. Therefore, Aquachlorobis(ethylenediamine)cobalt(II) chloride shows geometrical isomerism.

Option (A) Potassium tris(oxalato)chromate(III) does not show geometrical isomerism as it consists of only one type of ligand, oxalate.
Option (B) Pentaaquachlorochromium(III) chloride does not show geometrical isomerism as it has a octahedral structure with only one type of ligand, water.
Option (D) Potassium amminetrichloroplatinate(II) does not show geometrical isomerism as it has an octahedral structure with only one type of ligand, ammonia.

Therefore, the correct answer is option (C) Aquachlorobis(ethylenediamine)cobalt(II) chloride.