The correct answer is D. Six books were left on my desk by other students.
Step-by-step explanation:
In grammar, the passive voice is a type of construction that emphasizes the object affected by the action and the main action while the subject or agent responsible for the action is passive or is unknown. The passive voice can be recognized because the sentence begins with the object and it is followed by a past participle, also, it is common the subject is presented only at the end.
According to this, the only sentence that is in passive voice is "Six books were left on my desk by other students" because, in this, the emphasis is the objects "six books", it includes a past participle and auxiliary verb "were left" and the subject is passive as it is presented only at the end of the sentence.