Correct Answer:
C. had finished
The correct answer is C: had finished. This sentence requires the Past Perfect tense because it describes an action that was completed before another action or a specific point in the past. The phrase "before sunset yesterday" clearly indicates a past time marker, and the action of "finishing his work" happened prior to that specific past moment. The Past Perfect tense is formed with "had" + past participle (finished).
- A: finishes is Present Simple, used for habitual actions or facts.
- B: finished is Past Simple, used for completed actions at a specific time in the past, but it doesn't convey an action completed *before* another past event.
- D: has finished is Present Perfect, used for actions completed recently or with a result relevant to the present.
Therefore, "had finished" correctly places the completion of work before the specified past time.