They’re called multi-select questions and you’re warned about them by the professor ahead of time. Sometimes they have a “none of the above” option but sometimes they don’t.

The purpose is to get you to think critically. If you look at a question and all of the options seem wrong to you, making a guess because “one of them has to be right, it’s multiple choice” is lazy thinking and has nothing to do with the subject matter. In real life you don’t have the luxury of knowing that there’s one correct answer among a set of options.

The purpose of an exam is to test your knowledge of the course subject, not to test your ability to take multiple choice tests. Sometimes that’s hard to separate out, but the standard multiple choice question (where one and only one answer must be correct) is very game-able by guessers who don’t know the subject very well and risks the possibility of someone getting very lucky while not knowing anything.