Final answer:
Bacteria are living, single-celled organisms that can reproduce outside the body with appropriate nutrients. Protists are also single-celled eukaryotic organisms that don't fit into the categories of plants, animals, or fungi.
Step-by-step explanation:
The living, single-celled organisms that can grow and reproduce outside the body if given the appropriate nutrients are bacteria. Unlike viruses, which are obligate parasites and require a host to reproduce, bacteria can thrive and multiply in various environments provided they have the necessary nutrients. Bacteria belong to a category of life known as prokaryotes, which are distinguished by their lack of a membrane-enclosed nucleus. On the other hand, bats are multicellular animals, and parasites can be a variety of organisms, often multicellular, which live in or on a host organism and get food at the expense of their host.
Additionally, while discussing single-celled organisms that do not fit neatly into the categories of plants, animals, or fungi, it is worth mentioning protists. Protists can be similar to bacteria in being single-celled, but they are eukaryotic organisms, meaning they have a membrane-enclosed nucleus and other membrane-bound organelles. They may also exhibit characteristics similar to animals, such as hunting for food, or like plants, such as photosynthesis.