Which of the following is the smallest organism?

Viruses are indeed the smallest organisms when compared to the other choices provided. They typically range in size from about 20 to 300 nanometers, making them significantly smaller than bacteria, fungi, and protozoa.

Bacteria, while they can be very small, usually range from about 0.2 to 10 micrometers in size. Fungi consist of larger structures, such as molds and yeasts, which can be seen with the naked eye, and protozoa are typically single-celled organisms that are also larger than viruses, often measuring a few micrometers to several hundred micrometers.

Understanding the size of these organisms is important, not only in microbiology but also in fields such as medicine and environmental science, where the characteristics of pathogens and their effects on health are studied. Viruses, despite their small size, can cause significant diseases, highlighting the importance of considering their biological and medical relevance.