Which of the following is NOT a one-celled micro-organism?

A parasite is not classified as a one-celled microorganism because it typically refers to organisms that live on or in a host and derive nutrients at the host's expense. While some parasites, such as certain protozoa, are single-celled organisms, the term "parasite" encompasses a broader range of living beings, including multicellular organisms like helminths (worms).

In contrast, bacteria are single-celled organisms that can exist independently, viruses are acellular entities that require a host cell to replicate, and fungi can be unicellular (like yeast) or multicellular (like molds). Therefore, within the context of single-celled organisms, only viruses and bacteria fit that category directly, while fungi can fall into both categories. The choice of parasite is appropriate as it does not strictly adhere to the definition of a one-celled microorganism.