What term describes a living organism that obtains food from another organism?

The term that describes a living organism that obtains food from another organism is "parasite." Parasites depend on their host for sustenance, often causing some level of harm in the process. This relationship is characterized by the fact that the parasite derives its nutrients from the host, exploiting it for survival.

In contrast, the other terms refer to different types of ecological relationships. A mutualist describes an organism that interacts with another organism in a way that benefits both parties, such as bees pollinating flowers while collecting nectar. Commensal organisms benefit from their interaction with a host without affecting the host positively or negatively—an example being barnacles on a whale. A symbiont refers to an organism that lives in association with another, which could be mutualistic, commensal, or parasitic. Thus, the clear distinction lies in the specific nature of how the organism interacts with others, making "parasite" the correct term for an organism that exclusively obtains food at the expense of another.