What is a microorganism?

A microorganism refers to a very small living organism that typically cannot be seen without the aid of a microscope. This definition encompasses a broad range of organisms including bacteria, viruses, fungi, and some protozoa. The correct choice emphasizes that microorganisms are indeed living entities, and it highlights their microscopic size, which is central to their classification.

While the option mentioning just a "microscopic animal" may initially sound accurate, not all microorganisms are animals. For example, bacteria and fungi, which are also classified as microorganisms, do not fit the description of being animals. Additionally, some microorganisms can exist in forms other than animals, further elaborating the diversity within the category.

In terms of size, microorganisms are typically defined as being small enough to require a microscope for observation. The non-living particle option refers to viruses, which blurs the line since viruses exhibit characteristics of living things only when inside a host cell, making this distinction important. Hence, the understanding of microorganisms includes their living status and their microscopic nature, reinforcing why the highlighted answer aligns accurately with the established scientific definition.