What type of backup offers the best security against unauthorized access?

The option that focuses on fiber-optic cables for alternative routing offers a level of network security primarily associated with data transmission rather than the direct security of stored backup data. While using secure transmission methods like fiber-optic cables can help protect data in transit from interception, it does not inherently provide security for the actual stored backups against unauthorized access.

Full system backups, incremental backups, and disk-to-disk backups focus on the methods and the types of backups rather than the transport medium. Among these, a full system backup would typically ensure that all necessary data is backed up in its entirety, potentially in a more secure manner if managed properly—however, security against unauthorized access is contingent on how the backup media is protected.

Full system backups encompass all data and configurations, making restoration straightforward. This option implicitly suggests that secure access controls and encryption can be applied to the backup files, ensuring that only authorized personnel can access the backup information. Therefore, when managed with proper security practices, a full system backup can significantly enhance security against unauthorized access to sensitive data, making it a suitable option for addressing the primary concern of this question.