Final answer:
The sentence "The papers were graded by the teacher" is in passive voice.
Step-by-step explanation:
The sentence "The papers were graded by the teacher" is in passive voice. In this sentence, the subject of the sentence (papers) is receiving the action of the verb (graded) and the doer of the action (teacher) comes after the verb. The subject is not performing the action itself. In contrast, the sentences "The teacher returned the papers" and "The teacher graded the papers" are in active voice because the subject (teacher) is performing the action.
Learn more about passive voice in English grammar