The question asks about the purpose of using passive voice. 'D. To make the writing more formal' is the correct answer because passive voice is often used in formal writing to sound more objective. 'A. To emphasize the subject' is more related to active voice, 'B. To focus on the action' is also a feature of active voice, and 'C. To avoid responsibility' can be a side effect but is not the primary purpose of using passive voice.