The correct answer is Ill-.
A prefix is a part of a word that you add at the beginning, before the root word, whereas suffixes are added at the ending. The prefix inter- (as in intermediate), means in; the prefix dis- (as in disengage) means away, apart; the prefix en- (as in enable) means to cause something to happen.
Only the prefix ill- has a negative meaning. For example: illegal (the opposite of legal).
