Correct Answer: Who has done this mischief?

Solution : The correct choice is the third choice.

Explanation:
Active voice is a sentence or clause in which the subject performs the action. It typically follows the following structure: subject, verb, and object. The active voice is often more direct and straightforward than the passive voice.

In the given sentence, "By whom has this mischief been done?" the passive voice is used. To convert it to active voice, we need to identify the subject that performs the action (mischief) and restructure the sentence accordingly.
Conversion:
1. Identify the subject: "This mischief."
2. Identify the verb: "has been done."
3. Restructure the sentence in the active voice.

Therefore, the active voice would be: Who has done this mischief?