The correct answer is 'Why does a human being find it necessary to fight for freedom and justice?'. This is an example of a rhetorical question; the question is not meant to be given an actual answer, it is just posed to make the person reading or listening to it think about what is being asked. For instance, when you say 'Who knows?', you're not expecting an actual answer, you're just asking to emphasize your belief that no one really knows. Every other answer to this question is just asking for factual information, which has little to do with rhetoric unless it is being used as evidence in persuasive writing, which it is not in this example.
