Which of the following relationships is characterized by one organism benefiting at the expense of another?

The relationship characterized by one organism benefiting at the expense of another is known as parasitism. In this type of interaction, one organism, the parasite, gains advantages such as nourishment or habitat, while the host organism suffers some form of harm, which can range from mild discomfort to severe health issues or even death. This dynamic illustrates how parasitism represents a clear benefit to the parasite at the expense of the host.

The other terms reflect different ecological relationships. Commensalism involves one organism benefiting while the other is neither helped nor harmed. Mutualism describes a relationship where both organisms benefit from the interaction. Phoresis refers to a situation where one organism uses another for transportation, without directly benefiting at the expense of the other. Each of these interactions emphasizes a distinct type of relationship in ecology, but parasitism specifically highlights the detrimental effect on the host organism while providing benefits to the parasite.