Which of the following best describes a microorganism?

Microorganisms are defined as microscopic living organisms that can only be seen with the aid of a microscope. This category includes a wide variety of life forms such as bacteria, viruses, fungi, and protozoa. The key characteristic that distinguishes microorganisms is their small size, which allows them to inhabit a multitude of environments, including soil, water, and even within other living organisms.

When examining the other options, large animals are clearly not microorganisms because they can be seen with the naked eye and typically fall within the category of macroscopic organisms. Similarly, a type of plant does not fit the definition of microorganisms either, as many plants are much larger and are characterized by their photosynthetic capabilities. As for a chemical compound, this refers to a substance formed from two or more elements that are chemically bonded, which does not pertain to living organisms. Hence, the best description of a microorganism is indeed a small living organism.