Fill in the blanks with appropriate modals:

Ram works hard lest he…………………. fail.

should

The modal should is the most appropriate modal to fill in the blank. The modal should is used to express advice or a suggestion. In this case, the speaker is suggesting that Ram should work hard lest he fails because it is important for him to pass his exams.

The other options are incorrect.

• Must is used to express certainty or strong obligation. In this case, Ram does not have to work hard, but he should.
• Might is used to express possibility. In this case, the speaker is not expressing possibility that Ram will fail.
• Will is used to express certainty about something that will happen in the future. In this case, the speaker is not expressing certainty that Ram will fail.