What type of organisms are classified as one-celled?

Bacteria are classified as one-celled organisms due to their structure and function. They are prokaryotic cells, meaning they lack a nucleus and other membrane-bound organelles, and they exist as single cells capable of independent life. Bacteria can reproduce through binary fission, where one cell divides to form two identical daughter cells, which highlights their single-celled nature.

In contrast, viruses are not classified as one-celled organisms at all because they are not alive in the traditional sense and cannot reproduce or carry out metabolic processes independently. They require a host cell to replicate. Fungi, while some can be unicellular (like yeast), are primarily multicellular organisms (like molds and mushrooms) and have a more complex structure. Parasites encompass a wide variety of organisms, many of which are multicellular (like worms), although some, such as certain protozoa, are unicellular.

Thus, within the context of the choices provided, bacteria stand out as the clear representatives of one-celled organisms, characterized by their simplicity and independence as a single-cell life form.