What defines a microorganism?

A microorganism is best defined as a small living plant or animal that can only be seen with a microscope. This definition encompasses various types of organisms, including bacteria, viruses, fungi, and protozoa, all of which play significant roles in ecosystems, human health, and disease. Microorganisms are typically too small to be seen with the naked eye, making them unique in their visibility and scale. Understanding microorganisms is essential in fields such as microbiology, medicine, and environmental science, as these tiny life forms contribute to processes like fermentation, decomposition, and even human digestion.

The other options do not accurately define microorganisms. For example, stating that a large animal observable without a microscope defines a microorganism contradicts the fundamental aspect of size that is intrinsic to the definition. Similarly, describing a synthetic substance used in medical treatments does not pertain to living organisms at all, and a method of cleaning medical instruments relates to procedures rather than the organisms themselves. Therefore, the choice that identifies microorganisms accurately as small living entities visible only through a microscope is the most fitting.