Correct Answer: Who wrote these letters?

Solution : The fourth option is the correct answer.

In the original passive sentence, the subject is these letters, the verb is were written and the doer of the action is whom. In the active voice sentence, the doer of the action becomes the subject, resulting in the sentence, "Who wrote these letters?" in which who is now the subject, wrote is the verb, and these letters act as the object.

Here, the active voice sentence in the case of the interrogative simple present tense follows the structure: i nterrogative word + past form of the verb + object?

Therefore, the correct sentence is, Who wrote these letters?