What is a defining feature of parasites?

The defining feature of parasites relates to their biological characteristics and how they interact with their hosts. Parasites are typically eukaryotic organisms that can be unicellular or multicellular, often possessing specialized structures and organelles that enable them to live in specific habitats and interact effectively with their hosts.

The option that highlights varied cell sizes and being eukaryotic aligns with the diversity found within parasitic organisms. These can include complex multicellular organisms, such as worms, as well as simpler unicellular forms, like protozoa. Eukaryotic cells contain organelles, which are essential for the complex life processes that parasites must carry out in order to survive and reproduce within host organisms.

The other options do not accurately represent the nature of parasites. For instance, single-celled prokaryotic organisms do not fit the description, as parasites encompass eukaryotic organisms. Similarly, while eukaryotes do possess organelles, stating that parasites have "no organelles" contradicts the essential cellular structure found in these organisms. Lastly, the mention of "small cells, non-cellular structures" does not apply since many parasites, particularly multicellular ones, are not simply small and non-cellular; they have distinct cellular organization.