The correct answer is: should.
The word “lest” is a conjunction that means “in order to avoid” or “so as not to”. In this sentence, it is used to introduce a clause that expresses a negative consequence of not doing something. The verb “should” is used in this clause to express a strong recommendation or obligation. In other words, the sentence is saying that you should walk steadily in order to avoid stumbling.
The other options are not correct because they do not express a strong recommendation or obligation. The word “would” is used to express a possibility or likelihood. The word “could” is used to express ability or permission. The word “ought to” is used to express a moral obligation.
Here are some examples of how the word “lest” can be used in other sentences:
- I’ll wear a coat lest I catch a cold.
- You should be careful lest you get hurt.
- I’ll lock the door lest someone break in.