What is the definition of a microorganism?

The correct choice defines a microorganism as a minute one-celled form of life. Microorganisms are typically microscopic, which means they cannot be observed without the aid of a microscope. This category primarily includes single-celled organisms such as bacteria, archaea, and some fungi, as well as certain types of algae and protozoa. Their small size and simple structure enable them to thrive in diverse environments, making them essential to various ecological systems and human health.

Understanding this definition is crucial since it distinguishes microorganisms from larger, multicellular organisms—like animals and plants—that do not fit the criteria for classification as microorganisms. By emphasizing their single-cell nature and microscopic scale, the definition highlights the unique characteristics that set them apart in biological sciences.