The correct answer is option 'C' [Ni(NH3)2Cl2].

Explanation:
Isomerism is the phenomenon where two or more compounds have the same molecular formula but different arrangements of atoms. There are several types of isomerism, including structural isomerism, geometric isomerism, and optical isomerism.

In the given complexes, we need to determine which one is not expected to exhibit isomerism. Let's analyze each complex individually:

a) [Ni(NH3)2(H2O)2]2
This complex has two types of ligands - NH3 (ammine) and H2O (aqua). Since both ligands are different, they can occupy different positions around the central nickel (Ni) atom. Therefore, this complex is expected to exhibit geometric isomerism.

b) [Pt(NH3)2Cl2]
This complex also has two types of ligands - NH3 (ammine) and Cl (chloro). The two chloride ions can either be adjacent to each other (cis isomer) or opposite to each other (trans isomer). Therefore, this complex is expected to exhibit geometric isomerism.

c) [Ni(NH3)2Cl2]
This complex has two types of ligands - NH3 (ammine) and Cl (chloro). However, there are no other ligands to create different arrangements around the central nickel (Ni) atom. Therefore, this complex is not expected to exhibit isomerism.

d) [Ni(en)3]2
This complex has a bidentate ligand called en (ethylenediamine) which can form two bonds with the central nickel (Ni) atom. Since there are no other ligands present, the arrangement of en ligands will be fixed. Therefore, this complex is not expected to exhibit isomerism.

In summary, the complex [Ni(NH3)2Cl2] (option C) is not expected to exhibit isomerism because there are no other ligands to create different arrangements around the central nickel (Ni) atom.