What defines a parasite?

A parasite is defined as an organism that must live on a host to survive. This relationship is characterized by the parasite relying on the host for nourishment and shelter, often to the detriment of the host's health. The nature of this relationship typically involves the parasite extracting nutrients from the host, which can weaken or harm the host organism over time.

In contrast, organisms that can live independently do not require a host at all, which is why that option does not fit the definition of a parasite. The option relating to enhancing plant growth refers to beneficial organisms that may help plants thrive rather than harm them, which also diverges from the concept of parasitism. Lastly, while some organisms may have mutualistic relationships with their hosts where both parties benefit, parasites have a distinct relationship that is generally harmful to the host, further clarifying why those choices do not align with the definition of a parasite.