Explanation:
The correct answer is option 'C' - "He passed the examination in the first class because he had worked hard for it." Let's analyze why this answer is correct.

Understanding the sentence:
The sentence states that "He passed the examination in the first class." This means that he achieved a high score or grade in the examination. The reason for his success is then given in the second part of the sentence, which is "because he had worked hard for it."

Past perfect tense:
The use of the past perfect tense "had worked" indicates that the action of working hard was completed before the past event of passing the examination. It suggests that he put in the effort and dedication to prepare for the examination in the past, and this led to his success in achieving a high score.

Eliminating incorrect options:
Let's analyze the other options and understand why they are incorrect.

a) was hard working for it:
This option is grammatically incorrect. The correct form should be "he was working hard for it" or "he worked hard for it."

b) worked hardly for it:
This option is also grammatically incorrect. The word "hardly" means barely or scarcely, which does not convey the intended meaning. The correct form should be "he worked hard for it."

d) was working hard for it:
This option is grammatically correct, but it does not convey the cause and effect relationship between working hard and passing the examination. The use of the past perfect tense in option 'C' clearly indicates that the hard work was completed before the achievement of passing the examination.

Conclusion:
In conclusion, the correct answer is option 'C' - "He passed the examination in the first class because he had worked hard for it." This option accurately conveys the cause and effect relationship between the hard work and the successful outcome of passing the examination.