Which of the following organisms primarily engages in parasitism?

Fish lice are a type of crustacean that primarily engage in a parasitic lifestyle by attaching themselves to fish. They feed on the host's skin and bodily fluids, which can lead to irritation, stress, and even disease in the affected fish. Parasitism is a symbiotic relationship where one organism benefits at the expense of another, and fish lice exemplify this by relying on their fish hosts for nutrition and habitat.

In contrast, sea cucumbers are benthic marine animals that play a vital role in the ecosystem as detritivores, scavenging organic matter rather than engaging in parasitism. Corals, while they have mutualistic relationships with zooxanthellae (algae that live within their tissues), do not engage in parasitism themselves. Plankton refers to a diverse group of organisms, primarily tiny marine creatures, that often exist as free-floating organisms and do not have a parasitic lifestyle. Hence, fish lice stands out as the correct answer concerning organisms that primarily engage in parasitism.