What is an organism that lives on or within another living organism called?

The term you’re looking for is indeed a parasite. A parasite is an organism that lives on or in a host organism and derives nutrients at the host's expense. This relationship can harm the host in various ways, ranging from mild discomfort to severe health issues, depending on the type of parasite involved.

Understanding the broader context of the other terms can further clarify why parasite is the correct choice. A pathogen, while also harmful, generally refers specifically to microorganisms, such as bacteria or viruses, that can cause disease, but it doesn’t necessarily imply a dependency on a host for survival.

The terms paraplegic and paresis relate to conditions affecting bodily function rather than describing an organism's relationship with a host. Paraplegic refers to a person who has lost the use of their legs and lower body, while paresis describes a condition of weakness or partial paralysis. Neither of these terms addresses the biological relationship between organisms.

Thus, in the context of living organisms and their interactions, a parasite is specifically characterized by its dependence on a host for survival, clearly making it the most accurate term among the options provided.