What are protists?

Protists are classified as eukaryotic organisms, meaning their cells contain a nucleus and membrane-bound organelles. This classification allows for a significant diversity among protists, as they can exist as single-celled organisms, like amoebas and paramecia, or as multicellular forms, such as certain types of algae. The eukaryotic nature of protists distinguishes them from prokaryotic organisms, which do not have a nucleus, and accounts for the variety in their cellular structures and functions.

Additionally, they are not restricted to marine environments; protists can be found in freshwater, moist terrestrial environments, and even in symbiotic relationships with other organisms. Thus, the defining feature that identifies protists is their eukaryotic structure and the possibility of being either unicellular or multicellular, making C the correct characterization.