Correct Answer: By whom could such a silly mistake have been made?

Solution : The second option is correct.

Passive voice is the voice in which the object experiences an action rather than the person who performs the action. Hence, the object in the active voice becomes the subject in the passive voice.

The given sentence is in the active voice . The process of transformation is that the relative pronoun ( who) is changed to ( by whom) in the passive voice. Could have made will be changed into could have been made .

The structure of the sentence in the passive voice is: " By whom + past form of the modal verb + object + have been + past participle form of the verb?"

Therefore, the correct sentence is: By whom could such a silly mistake have been made?