The correct answer is immutability, which is a critical concept in backup security and ransomware resilience as covered in the CompTIA Security+ SY0-701 study guide. Immutability ensures that backup data, once written, cannot be altered, modified, or deleted for a defined period of time. This protection is essential in ransomware recovery scenarios because modern ransomware often attempts to encrypt or delete backups to prevent recovery.

Immutable backups are typically implemented using write-once-read-many (WORM) storage or immutable cloud storage configurations. When immutability is enforced, even administrators or attackers with elevated privileges cannot modify the backup contents during the retention window. As a result, organizations can be confident that their backups remain in a known-good, unaltered state, free from ransomware infection or tampering.

The other options do not provide the same guarantee. Destruction refers to permanently deleting data, which would eliminate backups rather than protect them. Sanitization is the process of securely erasing data from storage media and is unrelated to preserving clean backups. Retention defines how long backups are kept but does not protect them from being modified or encrypted during that period.

From a Security+ SY0-701 perspective, immutability is closely tied to resilience, recovery, and data protection strategies. It supports business continuity by ensuring that organizations can reliably restore systems after an attack. Immutable backups are a cornerstone of modern ransomware defense strategies because they prevent attackers from corrupting recovery data. Therefore, immutability is the best and most effective control to guarantee that backups used for recovery are not infected.