Which prefix means 'not'?

• A. Un-
• B. Re-
• C. Semi-
• D. Trans-

Answer: (A)

The main idea here is negation using a prefix. The prefix that means "not" is un-, which flips the meaning of a base word to its opposite or conveys the absence of something. For example, unhappy means not happy, unclear means not clear, and unplug means to take out the plug or disconnect. The other prefixes don’t express negation: re- means again or back, semi- means half or partly, and trans- means across or beyond. So un- is the best choice because it directly conveys negation.