This organism is not an example of eukaryotic cells

The correct option is Escherichia coli.

To understand why this is the correct answer, let's break down the classification of cells into eukaryotic and prokaryotic cells:

1. Eukaryotic Cells vs. Prokaryotic Cells:
- Eukaryotic cells are cells that contain a membrane-bound nucleus and other membrane-bound organelles (such as mitochondria, endoplasmic reticulum, and Golgi apparatus). Organisms containing these cells are called eukaryotes.
- Prokaryotic cells, on the other hand, lack a defined nucleus and membrane-bound organelles. Their genetic material is located in a nucleoid region that is not enclosed by a membrane. Organisms with these cells are called prokaryotes, which primarily include bacteria and archaea.

2. Analyzing the Options:
- Euglena viridis: This is a unicellular eukaryotic organism belonging to the kingdom Protista. It contains a distinct nucleus and membrane-bound chloroplasts.
- Amoeba proteus: This is a unicellular eukaryote (a protozoan) containing a well-defined membrane-bound nucleus and organelles like contractile vacuoles.
- Escherichia coli (E. coli): This is a species of rod-shaped bacterium. Because it is a bacterium, it is classified as a prokaryotic organism. It does not possess a nuclear membrane or any membrane-bound organelles.
- Paramoecium caudatum: This is also a unicellular eukaryote (a ciliated protozoan) containing organized nuclei (a micronucleus and a macronucleus).

Therefore, among the given choices, Escherichia coli is the only prokaryotic organism, making it the correct answer to the question of which organism is not an example of eukaryotic cells.