Antonyms are words that have contrasting, or opposite, meanings.
Choice C is correct. 'Prone to' means 'likely to experience something unpleasant' the opposite being 'immune to' meaning 'not influenced or affected by something'. For example: Babies have to be given shots so that they grow up immune to deadly diseases. Choice A means 'before something'. Choice C means 'before'. choice D means 'exposed to'. Choice A,B, and D are wrong.