Explanation
The correct answer is:
D) should
Explanation: The phrase "Work hard lest you should fail" is a correct construction. "Lest" is used to introduce a negative purpose or to express a warning of something undesirable happening. In this sentence, it suggests that by working hard, the person should avoid the possibility of failing. The word "should" is used here to indicate that the person is advised or expected to take action to prevent the undesirable outcome. Therefore, option D is the correct choice.
