He passed the examination in the first class because he ......

He passed the examination in the first class because he had worked hard for it.
In this context, the past perfect tense "had worked hard" is used to indicate that the action of working hard occurred before another past action (passing the examination). This implies that his hard work was completed before he achieved success in the examination. Therefore, "had worked hard for it" is the most appropriate choice to complete the sentence, conveying the sequence of events correctly.