Which of the following is the smallest form of microorganisms?

Viruses are considered the smallest form of microorganisms. They are unique in that they are not classified as living organisms outside of a host. Viruses consist of genetic material (DNA or RNA) surrounded by a protein coat and, in some cases, an outer lipid envelope. Their size generally ranges from about 20 to 300 nanometers, making them significantly smaller than bacteria, fungi, and protozoa, which are all cellular organisms.

Bacteria typically range from 0.5 to 5 micrometers, which is substantially larger than viruses. Fungi, such as yeasts and molds, are also much larger as they can be single-celled or multicellular, and their cells usually measure several micrometers in size. Protozoa are single-celled organisms that can range in size, but they are generally larger than both bacteria and viruses, often measuring tens to hundreds of micrometers. Therefore, the size comparison clearly establishes viruses as the smallest microorganisms among the given choices.