What best defines a parasitic relationship in biology?

A parasitic relationship in biology is best defined as one in which one organism lives on or within another organism, deriving benefits at the host's expense. This typically involves the parasite relying on the host for resources such as nutrition, shelter, or reproduction, which can often cause harm to the host.

This dynamic is distinctive because while the parasite is benefiting, the host may suffer from various negative effects, including disease, nutrient depletion, or even death, depending on the nature of the relationship and the type of organisms involved.

In contrast, mutual benefit signifies a relationship where both species benefit, predation involves one organism killing and consuming another, and commensalism indicates a relationship where one organism benefits without significantly affecting the other. Each of these forms of interaction highlights a different ecological relationship, but none align with the key characteristic of a parasitic relationship as accurately as living on another organism.