Rewrite the sentence in passive voice:

Who taught you this poem?

Option A is the correct answer. The two kinds of grammatical voices are active and passive. When the subject is the doer of the action in a sentence, it is in active voice, and when the subject is on the receiving end of action, the sentence is in passive voice. The first two options end with “by whom.” Questions that begin with “who” can be rewritten in passive voice using “by whom” in the beginning.