What is a small living thing that can only be seen with a microscope?

A small living thing that can only be seen with a microscope is accurately described as a microorganism. This term encompasses a broad category of organisms that are microscopic and cannot be seen with the naked eye. Microorganisms include various entities such as bacteria, viruses, and fungi.

By selecting this option, you're recognizing that microorganisms encompass a diverse group of life forms that share a common characteristic of being too small for direct observation without the aid of a microscope. This is crucial for understanding many biological processes and health-related topics, as microorganisms play significant roles in ecosystems, human health, and disease.

The other choices also describe small living things but do not capture the broader definition that the term "microorganism" provides. Viruses and bacteria are indeed types of microorganisms, while fungi also include microscopic forms. However, they are not all-encompassing categories like "microorganism," which is why it is the most accurate choice in this context.