Final answer:
Bacteria are single-celled microorganisms that are often referred to as germs or microbes. They are prokaryotic, which means they lack a nucleus, and can exist in diverse environments, sometimes causing diseases in humans and other organisms.
Step-by-step explanation:
The type of one-celled micro-organisms sometimes called germs or microbes is B. Bacteria.
Bacteria are single-celled microorganisms and a type of germ that can cause disease. These tiny living beings are prokaryotic, meaning they lack a nucleus, and can be found in virtually all environments on Earth. Some bacteria are beneficial, playing a role in processes such as digestion and the nitrogen cycle, while others can cause diseases. Unlike bacteria, viruses are not cells but are particles that require a host to replicate, and thus they are considered acellular.
Fungi and protozoa are also microorganisms that can cause disease; however, they are eukaryotic, possessing a nucleus within their cells. Fungi can be either unicellular, like yeasts, or multicellular, like molds, whereas protozoa are generally unicellular. It's important to note that while all these organisms can be microbes, the term "germ" is often specifically associated with bacteria due to their historical link to germ theory and infectious diseases.