What term describes parasites that live on or obtain nutrients from another organism?

The term that accurately describes parasites that live on or obtain nutrients from another organism is external. These parasites typically attach themselves to the host's body surface or reside in its environment, drawing nutrients directly from the host. This relationship is primarily one-sided, benefiting the parasite at the expense of the host.

In contrast, the other options reflect different types of interactions. Internal parasites would refer specifically to those that live within the host's body, such as tapeworms or roundworms. Mutualistic organisms engage in a relationship where both parties benefit, which does not apply to parasites since they typically exploit the host. Symbiotic relationships can include mutualistic scenarios but also encompass other interactions like parasitism, and does not specifically indicate the exploitative nature of the relationship.

Therefore, the definition of external as parasites that obtain nutrients from another organism is the most accurate in the context provided.