By whom are you taught English?
Here's why:
* Original sentence: "Who teaches you English?" (active voice)
* Subject: "Who" (the person teaching)
* Object: "you" (the person being taught)
* Verb: "teaches"
* Passive voice: The object ("you") becomes the subject, and the verb is changed to "are taught." The original subject ("who") becomes the object of the preposition "by."