The examples mentioned, Amoeba and guardian paramecium, belong to the group of organisms called protozoans. Protozoans are a diverse group of single-celled eukaryotic organisms that exhibit animal-like characteristics. They are classified under the kingdom Protista. Here is a detailed explanation of each option:

- Bacteria are single-celled prokaryotic organisms that have distinct characteristics different from protozoans.
- Bacteria lack a nucleus and other membrane-bound organelles.

- Viruses are non-living infectious agents that can only replicate inside the cells of other organisms.
- They are much smaller than protozoans and do not possess cellular structures.

- Protozoans are single-celled eukaryotic organisms that have a distinct nucleus and other membrane-bound organelles.
- They can be free-living or parasitic, and they exhibit animal-like characteristics.

- Fungi are a separate kingdom of eukaryotic organisms that includes multicellular organisms like mushrooms and molds.
- They have distinct characteristics different from protozoans.
Therefore, the correct answer is C: Protozoan. Amoeba and guardian paramecium are examples of protozoans, which are single-celled eukaryotic organisms with animal-like characteristics.