What is the definition of a microorganism?

A microorganism is defined as a living organism that is too small to be seen with the naked eye, which aligns perfectly with the chosen answer. This category of organisms includes bacteria, viruses, fungi, and protozoa, all of which play crucial roles in various ecological processes, biotechnology, and human health. Understanding this definition is important for anyone working in fields related to biology, health, or environmental science.

The other provided options do not accurately capture the essence of what a microorganism is. An organism that can be seen without a microscope is not classified as a microorganism. Additionally, while some microorganisms can reproduce outside living cells, this is not a defining characteristic of all microorganisms. Moreover, while there are organisms that exist only in extreme environments, like certain archaea, this description does not encompass the broad spectrum of microorganisms, many of which thrive in more common environments.