Concept:A possessive pronoun replaces a noun phrase like "our duty" so the noun is not repeated.Explanation:The sentence compares the teacher's duty with the duty of the speaker and others.The missing word must stand for "our duty"."Ours" is a possessive pronoun that can stand alone."Our" is a possessive adjective and needs a noun after it, such as "our duty"."Ours'" and "our's" are not correct standard forms.Thus, "ours" correctly completes the sentence.Answer:A. ours