Which of the following terms refers specifically to a method of system backup?

The term that refers specifically to a method of system backup is incremental backup. This type of backup focuses on saving only the data that has changed or been added since the last backup was taken. This approach is efficient because it reduces the amount of storage space required and minimizes the time needed to perform the backup. Incremental backups allow for quicker recovery times because you can restore your system to a specific point by combining the latest full backup with the subsequent incremental backups.

Other terms listed do not pertain specifically to a method of data backup. System Restore is a function that restores system files and settings to an earlier state but does not create a backup. Disk Cleanup is a utility that helps free up space on a disk by removing unnecessary files but does not involve creating backups. Fragmentation refers to the way data is stored on a disk and does not relate to backup processes. Therefore, incremental backup is the clear choice when discussing methods of system backup.