Which class of microorganisms includes the organisms that are typically not visible to the naked eye?

The choice indicating that all listed classes of microorganisms—fungi, bacteria, and protozoa—typically includes organisms that are not visible to the naked eye is correct.

Microorganisms are classified based on their size and cellular structure, often falling into these categories. Fungi can range from unicellular forms like yeast to larger structures such as molds, but the individual cells in these cases are usually microscopic. Bacteria are generally single-celled organisms that are also not visible without the aid of a microscope, as they typically measure only a few micrometers in length. Protozoa, being mostly unicellular eukaryotes, also fall into this category, with sizes that are often small enough to require microscopic examination.

Understanding that these various types of microorganisms share the characteristic of being typically microscopic helps establish the foundation for further study in microbiology, including how they affect human health and the environment.