What type of parasite is known as an obligatory parasite?

An obligatory parasite is defined as one that cannot survive or reproduce without a host organism. These parasites rely entirely on their host for nutrients and for completing their life cycle. This reliance often leads to a close relationship with the host, where the parasite may cause harm to the host in exchange for sustenance.

In contrast, the other options describe different types of relationships. A parasite that can live independently from its host describes a free-living parasite, which does not fit the definition of an obligatory parasite. A parasite that completes its life cycle only under specific conditions could refer to various types of parasites, including those with complex life cycles that require environmental factors or specific hosts but doesn't imply that they are obligatory. Lastly, a parasite that alternates between a free-living and a parasitic state describes a facultative parasite, which can exist independently in the environment, unlike obligatory parasites that must be within a host to thrive.