What is a prefix that means "not or opposite"?

The correct choice is a prefix that meaning "not or opposite" is "im." This prefix is commonly added to words to convey negation or an opposite meaning. For example, when "im" is added to "possible," it creates the word "impossible," which means something that cannot happen or is not possible. Similarly, it can be seen in other words like "imperfect" or "immature," where the prefix indicates the opposite quality of the root word.

The other options do not convey the meaning of "not" or "opposite." "Con," for instance, typically means "with" or "together." "Pre" refers to something that occurs before something else. "Anti" does mean against or opposite, but it does not fit under the specific context of meaning "not," which is why "im" is the most appropriate choice in this case.