What is a parasitic organism that cannot survive independently of its host called?

An obligate parasite is defined as a parasitic organism that depends entirely on its host for survival and reproduction. These organisms have specialized adaptations that allow them to exploit the host’s resources, often at the host's expense. Obligate parasites cannot complete their life cycle without being within or on a suitable host organism; they are entirely reliant on the host's biological systems for nutrients, shelter, and often reproduction.

In contrast, facultative parasites can lead an independent existence and are not strictly dependent on a host for survival, meaning they can survive outside of a host under certain conditions. Saprophytes are organisms that obtain their nutrients by decomposing organic matter, primarily dead or decaying organisms, rather than living hosts. Endoparasites are a category of parasites that live inside the host organism, but they can be either obligate or facultative. Thus, the term obligate parasite specifically identifies those organisms that cannot survive at all without the host, which makes it the correct choice in this context.