Answer:
The door was opened by who?
Step-by-step explanation:
The sentence in the question is denoted in the active voice. The active voice is that wherein the subject performs the action on the receiver.
The revised sentence above is in the passive voice. Here, the subject receives the action. The subject in this case is the pronoun 'who'. At first, it performed the action of opening the door. In the revised voice above, it receives the action of opening the door. The change in the sentences above show the differences between the active and passive voice.