Which description fits a microorganism?

A microorganism is defined as any organism that is microscopic or submicroscopic in size. This includes a wide range of living entities such as bacteria, viruses, fungi, and protozoa, many of which are not visible to the naked eye and require a microscope for observation. This definition encompasses both harmful and beneficial microorganisms, as not all microorganisms are pathogenic.

The other descriptions are more restrictive or inaccurate. For instance, visible organisms do not fit the definition of microorganisms because they includes larger entities that can be seen without special equipment. Similarly, limiting the definition solely to organisms that cause disease excludes beneficial microorganisms that play essential roles in ecosystems and human health. Additionally, stating that only bacteria are microorganisms is inaccurate, as it ignores the existence of other groups such as viruses and fungi. Thus, the broad definition of microscopic or submicroscopic size encompasses the diverse world of microorganisms effectively.