Explanation
The best approach for handling personal data that has been restricted is to flag users' email addresses to make sure they do not receive promotional information, because this will respect the users' preferences and rights to opt out of marketing communications. This will also help the company comply with the data protection laws and regulations that require consent and transparency for sending marketing emails, such as the General Data Protection Regulation (GDPR) and the CAN-SPAM Act12. The other options are not appropriate or sufficient for handling restricted data, because they may violate the users' rights, expectations, or agreements, or cause operational issues for the company.
References:
* CDPSE Review Manual, Chapter 3 - Data Lifecycle, Section 3.1 - Data Classification3.
* CDPSE Certified Data Privacy Solutions Engineer All-in-One Exam Guide, Chapter 3 - Data Lifecycle, Section 3.2 - Data Classification4.