Correct Answer - Option 2 : you should fall down.
The correct answer is 'you should fall down'.
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'Lest' in the given sentence is a conjunction.
- Some conjunctions come in pair.
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'Should' is always followed by conjunction 'lest'.
- If you do something lest something unpleasant should happen, you do it to try to prevent the unpleasant thing from happening.
- for example:
- I was afraid to open the door lest he should follow me.
- Thus, option 2 will be the correct answer.
Hence, the correct sentence is: Walk carefully lest you should fall down.