What type of organisms are primarily unicellular and can exist in both free-living and parasitic forms?

The correct answer is protista, which encompasses a diverse group of primarily unicellular organisms. This category includes various subgroups like protozoa and algae, which can exist in both free-living and parasitic forms. For example, protozoa like Plasmodium can cause diseases such as malaria, while others may thrive independently in various aquatic environments. This adaptability allows protista to occupy various ecological niches.

Bacteria are also unicellular but do not typically fit within this question's context of being classified as both free-living and parasitic in the same way as protista. Viruses, on the other hand, are not considered living organisms in the traditional sense, as they require a host cell to replicate and cannot exist independently. Fungi, while they include unicellular forms like yeasts, are primarily known for their multicellular structures and are mainly decomposers rather than free-living or parasitic.

Understanding the characteristics of protista is essential for recognizing their role in ecosystems and their impact on human health, especially considering their dual lifestyles.