Explanation
The subject line of the proof email when received by the recipient will be [Proof] Last chance to save 40% in the Sale. This is because when sending a proof email, Adobe Journey Optimizer automatically adds [Proof] as a prefix to the original subject line of the email message. This helps distinguish a proof email from a regular email and avoid confusion or spam reports from recipients. The other options are not valid or accurate subject lines of the proof email, as they do not include the [Proof] prefix or they include an extra Proof word.
References:https://experienceleague.adobe.com/docs/journey-optimizer/using/email-channel/design-your-email-c