Concept:A tag question is a short question added to the end of a statement to seek confirmation or agreement.
Explanation:When the main statement uses a positive auxiliary verb, the tag question must use the same auxiliary in its negative form.
Here, the statement is "John should be through with his work."
The auxiliary verb is "should," and it is positive.
Therefore, the tag should be "shouldn't he?"
The pronoun "he" refers to John, and "shouldn't" is the negative form of "should."
So the correct completion is "shouldn't he."
Answer:C. shouldn't he