Which microorganism requires a living host to survive and is smaller than bacteria?

The correct answer is a virus. Viruses are unique microorganisms that possess characteristics of both living and non-living entities. They are much smaller than bacteria and cannot reproduce or carry out metabolic processes outside of a living host cell. Their survival hinges entirely on the ability to infect a host, where they hijack the host's cellular machinery to replicate and propagate.

In contrast, protozoa and fungi are classified as living organisms that can often survive without a host and can reproduce independently under suitable environmental conditions. While protozoa are single-celled organisms that can live in various environments, fungi are a kingdom of their own, including yeasts and molds, and can thrive in diverse ecosystems. Thus, the only microorganism in the options listed that requires a living host to survive is a virus, making it the most appropriate choice.