What is the definition of a parasite?

A parasite is defined as an organism that lives in or on another organism, known as the host, and derives benefits at the host's expense. This relationship often results in harm to the host, which can manifest in various ways such as nutrient depletion, disease, or weakening of the host's immune system.

Option B specifically highlights that protists, which are a type of microorganism, can function as parasites when they inhabit other organisms and cause harm. This aligns with the broader definition of parasitism, which encompasses various types of organisms, including protists, fungi, and worms, beyond just viruses or bacteria.

In contrast, the other options define organisms in a way that does not align with the accepted concept of parasitism. For instance, the notion that a parasite is a type of virus limited to plant hosts is too narrowly focused, as parasites can be found across various biological categories and host types. Enhanced growth of a host also contradicts the fundamental characteristic of parasites, as they typically harm the host rather than support its growth. Lastly, organisms that do not require a host for survival cannot be considered parasites, as the key feature of parasitism involves dependence on a host.