A microorganism is any ___

• A. Organism of microscopic size
• B. Large organism
• C. Inorganic particle
• D. Bacteria only

Answer: (A)

A microorganism is any living thing that is too small to be seen without a microscope. The word micro- means small, so the defining feature is microscopic size. Large organisms are visible to the naked eye, so they aren’t microorganisms. An inorganic particle isn’t living, so it doesn’t fit. Saying bacteria only is too narrow because many other microscopic living forms—like fungi, protozoa, and algae (and sometimes viruses, depending on classification)—also qualify.