Which of the following is a type of single-celled micro-organism?

The best choice for a type of single-celled micro-organism is bacteria. Bacteria are prokaryotic, single-celled organisms that can be found in various environments, including soil, water, and even inside the human body. They play crucial roles in processes such as decomposition and nutrient cycling, and some can be pathogenic, causing infections and diseases in humans, animals, and plants. Understanding bacteria is essential in food safety management, as they can cause foodborne illnesses if food is not handled, cooked, or stored properly.

Fungi, while also a kingdom of life, typically include multicellular organisms like molds and mushrooms, although some fungi, such as yeast, are single-celled. Viruses are not classified as living organisms as they require a host to replicate and are not composed of cells. Parasites, on the other hand, are often multicellular organisms that depend on other living hosts for survival and nutrition; they do not function as single-celled entities. This context helps clarify why bacteria is the correct response in distinguishing between the different types of micro-organisms stated in the question.