Which of the following backup methods would take the LEAST amount of time to complete?

An incremental backup backs up only the data that has changed since the last backup (either full or incremental). This means the size of the incremental backup is typically very small, leading to the shortest backup window among the options for a single subsequent backup operation.

"All differential backups" is an incomplete strategy; it implies multiple differential backups, each growing larger as they include all changes since the last full backup, and a single differential backup might be larger than a single incremental.

A differential backup backs up all data that has changed since the last full backup, meaning over time, differential backups grow in size, potentially becoming quite large and taking longer than a daily incremental backup.

Alternating differential and incremental backups would involve both types, and while incremental backups are fast, a single subsequent incremental backup is generally the fastest option compared to a differential or an alternating scheme which might include a larger differential.