Which of the following prefixes means "thick"?

The correct choice is indeed the prefix that means "thick." The prefix "pachy/o" is derived from the Greek word "pachys," which translates to "thick." This prefix is commonly used in medical terminology to describe conditions or characteristics involving thick tissues or structures. For example, "pachycephaly" refers to an abnormal thickness of the skull.

The other choices represent different concepts entirely. "Py/o" is related to pus. "Rube/o" is associated with the color red, often used in terms like "rubella." "Pil/o" refers to hair. Understanding these prefixes helps to decode medical terminology effectively by recognizing the meaning they add to root words.