Concept:Possessive pronouns replace a noun phrase and show ownership without using an apostrophe.Explanation:The sentence compares the teacher's duty with the duty belonging to us.The phrase “our duty” can be replaced by the possessive pronoun “ours”.Option A “our” is a possessive adjective and needs a noun after it, such as “our duty”.Options B and C use apostrophes incorrectly because possessive pronouns never take an apostrophe.Therefore, after the semicolon, the correct subject is “ours”, meaning “our duty”.Answer:D. ours