* "Who broke it?" is correct when asking who is responsible for something that is now broken. This is in the past tense, referring to an action that has already happened.
* "Who break it?" is grammatically incorrect. It mixes tenses and doesn't make sense.
* "Who breaks it?" is correct when asking who usually or habitually breaks something. This is in the present tense, referring to a repeated action.
Here's an example of each:
* "Who broke the vase?" (Asking about a past event)
* "Who breaks the most toys?" (Asking about a habitual action)