Explanation:

- The sentence suggests avoiding negative influences to prevent future regrets.

- Option 4: "lest"

- "Lest" means to avoid the risk of something undesirable happening.

- Correct Answer: "Keep away from bad company lest you should regret later."

- Option 1: "whether"

- "Whether" expresses doubt or a choice between alternatives.

- Not suitable as it does not convey prevention of a negative outcome.

- Option 2: "unless"

- "Unless" is used to introduce an exception.

- Not appropriate as it doesn’t fit the context of avoiding future regret.

- Option 3: "although"

- "Although" shows contrast.

- It doesn't fit the message of prevention in this context.