Which prefix would be used to describe a process that occurs through or completely?

The prefix that describes a process occurring through or completely is "dia." This term originates from Greek, meaning "through," "across," or "completely." In medical terminology, it often indicates a thorough or complete process, such as in "diagnosis," which refers to the complete understanding of a condition or "diabetes," describing a full metabolic disorder.

On the other hand, "pre" generally indicates something that occurs before, "peri" refers to surrounding or around something, and "sub" implies something that is beneath or under. These prefixes do not convey the idea of thoroughness or completeness as "dia" does. Thus, "dia" is the most appropriate choice for describing a process that occurs through or completely.