Correct Answer:
C. had worked
This question requires the correct tense to describe an action that was completed over a period of time *before* another specific action in the past. The phrase "for five years" indicates a duration, and "before he resigned" sets a clear past reference point.
The past perfect tense (had + past participle) is used to show that an action was completed before another action or a specific point in the past. It helps to establish the sequence of past events.
- C: had worked is the correct answer. "He had worked in this company for five years before he resigned" correctly indicates that the action of working for five years was completed *prior to* his resignation (which is in the past simple). This clearly shows the sequence of events.
- A: worked is incorrect. This is the past simple. While grammatically possible, it doesn't emphasize the completion of the five-year period *before* the resignation as clearly as the past perfect does. The past perfect is more precise for this kind of sequence.
- B: has worked is incorrect. This is the present perfect, which connects a past action to the present. Since "he resigned" is a past event, the action of working must also be entirely in the past relative to that resignation.
- D: works is incorrect. This is the present simple tense, which is inappropriate for describing a past event.