Which of the following defines single-celled microorganisms that have both plant and animal characteristics?

The correct definition for single-celled microorganisms that exhibit both plant and animal characteristics is associated with protozoa. These organisms are a diverse group of single-celled eukaryotes that can mimic the characteristics of both plants and animals. For instance, some protozoa are autotrophic, like plants, because they can perform photosynthesis, while others are heterotrophic, like animals, as they consume other organisms for nutrition.

In contrast, bacteria are typically unicellular organisms that lack a distinct nucleus and are fundamentally different from both plants and animals. They do not have the complex structures typical of eukaryotic cells. Viruses, on the other hand, are not classified as living organisms and require a host cell to replicate, whereas fungi are multi-cellular organisms (except for yeasts, which are single-celled) that primarily decompose organic matter and have characteristics distinct from both plants and animals. Thus, the unique characteristics of protozoa align with the description given in the question.