What are parasites?

Parasites are defined as organisms that derive their nutrition and sustenance from other living entities, typically at the host's expense. This relationship often results in harm to the host, as parasites depend on their hosts for food and resources while potentially causing diseases or other negative effects. For example, common parasites include tapeworms and certain protozoa, which live inside the bodies of their hosts.

While the other options mention interesting types of organisms, they do not accurately describe the essence of what a parasite is. Microorganisms that can live in extreme conditions, for instance, refer more specifically to extremophiles and do not encompass the parasitic relationship. Organisms that provide nutrients to others are more aptly described as mutualists or symbionts, demonstrating a mutual benefit rather than a detrimental relationship. Lastly, genetically engineered organisms relate to biotechnology and do not reflect the definition of parasites, which are based on natural biological relationships rather than human-altered genetics.