Answer: The sentence that contains an appositive is A. The poet Keats studied to become a doctor.
Step-by-step explanation: An appositive is a noun or pronoun that renames another noun or pronoun which comes immediately before or after the appositive. In sentence A., the appositive is "Keats", which is a proper noun renaming the noun that precedes it: "poet". In most cases, appositives are separated from the rest of the elements of the sentence by means of a comma; however, in sentence A, the appositive has not been placed between commas. In other words, the appositive in A. is restrictive because the information provided by it is essential: if the appositive is removed, the meaning of the sentence changes.