Which type of microorganism is primarily unicellular and can be found in moist environments?

The correct answer is protozoa, as they are primarily unicellular microorganisms that thrive in moist environments. Protozoa belong to the kingdom Protista and are characterized by their complex cellular structure and the ability to carry out all necessary life processes within a single cell. They can be found in a variety of aquatic environments, such as ponds, rivers, and soil that retains moisture, where they play important roles in the ecosystem, including as decomposers and as a part of food webs.

Bacteria, while also primarily unicellular and found in moist environments, are a different group of microorganisms and encompass a much broader range of habitats beyond just moist areas. Viruses, on the other hand, are not classified as living organisms since they require a host cell to replicate and do not have cellular structures. Fungi generally comprise multicellular organisms (like mushrooms) but can also include unicellular forms (like yeast); however, fungi are more commonly associated with terrestrial environments rather than solely moist aquatic ones. This distinction reinforces why protozoa is the most suitable choice given the specific emphasis on being unicellular and their prevalent habitat in moist environments.