What type of backup identifies only the data modified since the last backup?

The type of backup that identifies only the data modified since the last backup is known as an Incremental backup. In this backup strategy, only the changes made since the last backup—whether it was a full backup or another incremental backup—are saved. This means that each incremental backup depends on the previous backup for data restoration.

Differential backups, on the other hand, capture all changes made since the last full backup, not just the last backup, leading to larger backup sizes over time. Full backups save all the data every time they are run, irrespective of any changes, while mirror backups create a copy that reflects the current state of the data, similar to a full backup but without historical versions.

Therefore, an incremental backup is the most efficient in terms of storage and time when only needing to identify and store changes, making it the correct answer.