Which prefix indicates a relationship "between"?

The prefix that indicates a relationship "between" is "inter-." This prefix derives from the Latin word "inter," meaning "between" or "among." It is commonly used in various terms within different fields, such as "interstate" (between states) and "interpersonal" (between people). This clarity of meaning contributes to its frequent application in medical and scientific terminology, reinforcing the idea of connections or interactions occurring between entities.

In contrast, the other prefixes do not convey this specific relational meaning. "Endo-" refers to "within" or "inside," "exo-" signifies "outside" or "external," and "hypo-" means "under" or "below." These definitions illustrate why "inter-" is the correct choice for indicating a relationship "between."