The correct choice is 'して' (te-form of 'する' - to do). Paired with the auxiliary verb 'くれました' (did the favor of), 'してくれました' means they did the favor of supporting/cheering. Sentence context: '...classmates waved their hands and supported me.' 'させて' (causative), 'されて' (passive), and 'しなくて' (negative te-form) do not match the benefactive '〜てくれる' structure.