Answer: The sentence that uses "turned" as a linking verb is the first one: "The milk turned sour after sitting out on the counter overnight".
Explanation: A linking verb is a verb that connects the subject of a sentence to a word or group of words that provide additional information about it. Therefore, linking verbs do not indicate an action and they are usually followed by an adjective. The verb "turn" can be a linking verb, but it can also be used as a transitive verb and an intransitive verb. In the sentence "The milk turned sour after sitting out on the counter overnight", "turn" has been used as a linking verb since it is connecting the subject ("The milk") to an adjective that describes it ("sour"). In other words, "turn" is not showing an action: it is establishing a relationship between two elements. In contrast, in the other two sentences ("She turned the doorknob but the door was locked" and "Have you turned in your writing assignment yet?"), "turned" has been used as a transitive verb.
