The passive voice of "Who broke this window?" is The window was broken by whom?
Passive voice is a construction in which the subject of a sentence receives the action of the verb rather than performing the action.
So, the subject of the active voice sentence ("Who") becomes the object, and the object of the active voice sentence ("this window") becomes the subject.
The verb "broke" is changed to its passive form "was broken," and the agent of the action (the person who broke the window) is omitted or placed at the end of the sentence. In this case, the agent is "by whom," making the sentence more formal.
The full question is:
Change the following sentence into passive voice:
Who broke this window?