What issue related to mail security involves preventing unauthorized access?

The correct answer addresses a critical concern in mail security: preventing unauthorized access. This issue specifically involves ensuring that mail and parcels are not opened or tampered with by unauthorized individuals. By preventing unauthorized opening, the integrity and confidentiality of the mail are preserved, which is essential for maintaining trust in postal services.

When considering the context of mail security, preventing unauthorized opening is important as it protects sensitive information and personal communications contained within the mail. This includes letters, financial documents, and packages that may hold valuable contents. Effective security measures, such as tamper-evident packaging and secure delivery practices, are designed to eliminate the risk of unauthorized access, thereby safeguarding the sender's and recipient's rights.

In contrast, other issues like theft refer to the actual removal of mail or parcels without permission and destruction pertains to intentional damage of mail items. While all these concerns are significant in the realm of mail security, unauthorized opening specifically highlights the focus on access control, which is pivotal in preserving the confidentiality and security of mail.