Which of the following is an example of a single-celled organism?

A bacterium is indeed an example of a single-celled organism, which plays a crucial role in various ecosystems and can be found in numerous environments, including extreme conditions. Bacteria are classified as prokaryotic organisms, meaning they lack a defined nucleus and membrane-bound organelles, distinguishing them from more complex life forms.

Fungi and algae can exist as single-celled organisms but are often multicellular. Fungi are typically more complex in structure, with multi-cellular forms like mushrooms being well-known examples. Algae, on the other hand, can range from single-celled organisms, like certain species of green algae, to large multicellular forms, such as seaweeds. Viruses, while considered biological entities that can infect host cells, do not fall into the category of living organisms, as they cannot carry out metabolic processes or reproduce independently without a host.

Thus, the identification of a bacterium as the correct answer effectively highlights the distinct characteristics of single-celled organisms in the wider context of cellular life.