Which of these prefixes refers to "across" or "through"?

The prefix that refers to "across" or "through" is indeed "trans-." This prefix comes from Latin, where it literally means "to go across" or "to pass through." In medical terminology, it is commonly used to indicate processes or conditions that involve crossing over or moving from one side to another. For example, in words like "transfusion," it implies the movement of blood across from one individual to another.

The other options represent different meanings: "sub-" means "under" or "below," "peri-" refers to "around" or "surrounding," and "dys-" denotes "bad" or "difficult." Each of these prefixes carries distinct implications that do not pertain to the concept of "across" or "through," thus reinforcing the accuracy of "trans-" as the correct choice.