Which prefix means "abnormal"?

The prefix that means "abnormal" is indeed "dys-." This prefix is commonly used in medical terminology to indicate a condition that is abnormal or difficult, often relating to the function of a bodily system. For example, "dysfunction" refers to an abnormal or impaired function of an organ or system.

In contrast to the other options, "ad-" typically means "toward" or "in the direction of," which conveys a positive or normative connotation rather than an abnormal state. "Dia-" means "through" or "across," often used to describe processes or conditions that are direct or complete. Finally, "ab-" means "away from," which denotes a position or direction away from something rather than indicating that something is abnormal. Therefore, "dys-" is the prefix that specifically captures the essence of abnormality in medical terminology.