What is the meaning of the prefix un-?

The prefix "un-" means "not." This prefix is commonly used in the English language to indicate negation or the opposite of a term it precedes. For instance, in words like "unhappy" or "undo," the prefix transforms the meaning of the root word by introducing a sense of negation, effectively creating words that convey a lack of or opposite quality. This usage is widespread and helps in forming antonyms, making it a fundamental aspect of understanding many English terms.

The other options provided represent different prefixes with their own distinct meanings. For example, "only" would suggest exclusivity, "without" indicates absence or lack, and "against" implies opposition. Each has its own unique application in language, but they do not relate to the negation that "un-" encapsulates.