What are microorganisms typically characterized as?

Microorganisms are typically characterized as single-celled microscopic organisms, which is why this answer is correct. Microorganisms include a diverse range of organisms, such as bacteria, archaea, protozoa, algae, and some fungi, most of which are composed of a single cell. Their small size allows them to be observed only under a microscope, making them distinct from larger multicellular organisms.

The other options do not accurately capture the essence of microorganisms. Although colony-forming units could be a characteristic relevant to certain microorganisms that grow in clusters (like some bacteria), it doesn't define what microorganisms are as a whole. Multicellular life forms refer to organisms that consist of multiple cells, which is not representative of the majority of microorganisms. Non-living particles, such as viruses, are not classified as microorganisms because they do not possess cellular structures and cannot carry out metabolic processes independently. Thus, the defining feature of microorganisms as single-celled organisms makes this answer the most appropriate.