Which of the following is not a common backup type?

• A. Full backup
• B. Incremental backup
• C. Extension backup
• D. Differential backup

Answer: (C)

Understanding common backup types and how they work helps you see why only some terms appear in practice. A full backup copies everything you want to protect in one go, providing a complete restore point but using more storage space and time. An incremental backup saves only the data that has changed since the last backup of any kind, which makes each run fast and saves storage, but restoring can require chaining many backup sets. A differential backup captures all changes since the last full backup, so restores are quicker than with increments because you only need the last full backup plus the latest differential, at the cost of growing storage use over time.

An extension backup isn’t a standard, widely recognized category in typical backup strategies. It isn’t part of the usual trio that defines how data is backed up and restored, and you won’t see it as a primary type in most environments. Some vendors or teams might use nonstandard terminology or describe additional steps that extend or augment a backup process, but it’s not a conventional backup type like the other three.