The correct option is Incremental backup.

Incremental backup copies only the data that has changed since the last backup operation. This means each incremental job records only new or modified files or blocks and therefore uses the smallest amount of data per run when compared with full or differential methods.

Differential backup is not correct because a differential copies all changes since the last full backup. Each differential backup therefore grows larger over time until another full backup is taken which makes it larger than an incremental job in most cases.

Full backup is not correct because a full backup records the entire dataset every time. That produces the largest amount of data for each backup operation even though full backups simplify restore processes.

Compute Engine snapshots is not selected because snapshots are image level captures and their behavior depends on the implementation. Some snapshot systems store only changed blocks after the initial snapshot which can be efficient. However snapshots are not the standard answer when the question asks which backup approach records the smallest amount of data per backup job and exams typically expect the term incremental backup for this characteristic.

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