Select the most appropriate option to fill in the blank.

Keep away from bad company ______ you should regret later.

lest

The most appropriate option to fill in the blank is lest.

The word "lest" is used to express a negative consequence. In this sentence, the speaker is warning the listener to stay away from bad company, lest they regret it later.

The other options are not grammatically correct or do not convey the same meaning as "lest."

• "Whether" is used to introduce a question. In this sentence, the speaker is not asking a question.
• "Unless" is used to introduce an exception. In this sentence, the speaker is not making an exception.
• "Although" is used to introduce a contrast. In this sentence, the speaker is not contrasting two ideas.