Explanation:

- The offer is good. You will regret unless you decline it.

- This implies you will have regret only if you don’t decline, which is not logical.

- The offer is good. You will regret whether you decline it.

- This suggests you will regret in both scenarios, declining and not declining, which contradicts the context.

- The offer is good. You will regret if you decline it.

- This means you will feel regret when you choose to decline. It clearly expresses the cause-effect relationship.

- The offer is good. You will regret therefore you decline it.

- This is incorrect as "therefore" is used to express a logical result, which doesn’t fit the sentence context.