Which prefix conveys the meaning of accompanying or supplementary?

The prefix that conveys the meaning of accompanying or supplementary is "para-." This prefix is derived from Greek, where it means "beside" or "alongside." In medical terminology, "para-" is often used to describe conditions or situations that are related to or occurring alongside a primary condition. For example, in words like "parainfluenza," it indicates a condition that is similar to or accompanying influenza.

In contrast, "post-" implies something that occurs after or behind, "super-" suggests something that is above, beyond, or in excess, and "mid-" indicates a middle position. Each of these prefixes creates entirely different meanings that do not convey the idea of being supplementary or accompanying, which is the primary reason "para-" is the correct choice in this context.