What type of microorganism can exist in a single-celled form?

Bacteria are indeed classified as single-celled microorganisms, which makes them the correct answer for this question. They are prokaryotic cells, meaning they do not have a nucleus or other membrane-bound organelles. This simplicity allows them to reproduce rapidly and adapt to various environments.

While fungi, viruses, and protozoa also play significant roles in the microbial world, they differ in their characteristics. Fungi can exist as single-celled organisms (like yeasts), but they are more commonly known as multicellular organisms (like molds). Viruses are distinct in that they are not considered living organisms; they require a host cell to replicate and do not possess cellular structures at all. Protozoa are another group of single-celled organisms, but they are eukaryotic and more complex than bacteria.

In summary, bacteria are the only microorganisms in this list that consistently exist in a single-celled form as prokaryotes, making them the most suitable answer to the question.