What constitutes an external parasite?

An external parasite is defined by its location and interaction with the host. These parasites reside on the outer surface of a host organism, where they attach themselves and draw resources, such as nutrients and blood, from the host's body. Examples of external parasites include fleas, ticks, and mites, which can cause harm to their hosts by feeding on their blood and potentially transmitting diseases.

The other choices describe different types of organisms or interactions. For instance, an internal parasite lives within the host, such as tapeworms or roundworms, which can cause harm from inside the host's body. A parasite that relies on the host for nutrients can refer to both internal and external parasites, but does not specify the residing location critical to defining external parasites. Finally, a parasite that attacks plants focuses on plant pathogens, which are not relevant in the context of animal hosts. Thus, the definition of an external parasite is accurately captured by the option that states it resides on the outside of a host.