Explanation:

- "Heads up!" is an alert or warning to pay attention or be on guard.

- Option 1: Rest well! - This means to take time to relax and recover, not the intended meaning of "Heads up!".

- Option 2: Look after yourself! - This suggests self-care and ensuring personal well-being, which isn’t the same as "Heads up!".

- Option 3: Watch out! - This is a warning to be cautious or alert, closely matching the meaning of "Heads up!".

- Option 4: Be careful! - While also a warning, it suggests being cautious, which is slightly different from the spontaneous alert nature of "Heads up!".

- Correct Answer: Option 3, Watch out! - This matches "Heads up!" as both convey a warning to be aware of something.