Which type of relationship is characterized by one species benefiting at the expense of another?

The correct answer is B, which refers to parasitism. In parasitism, one organism, the parasite, benefits by deriving nourishment at the expense of another organism, known as the host. This relationship is detrimental to the host, which may suffer health issues, reduced fitness, or even death as a result of the interaction. Classic examples include ticks feeding on mammals, or tapeworms living in the intestines of hosts.

In contrast, commensalism describes a relationship where one species benefits while the other is neither helped nor harmed. Mutualism involves both species benefiting from the interaction, such as bees pollinating flowers while obtaining nectar. Cooperation refers to a relationship in which both species work together to benefit each other, but it is not characterized by one benefiting at the other's expense. Thus, parasitism specifically highlights the dynamic of one species gaining at the detriment of another, making it the accurate choice in this context.