The repositories should be configured as Scale-out Backup Repositories consisting of hardened repositories to easily allow for additional capacity to be added and to meet the immutability requirement. Scale-out Backup Repository is a logical entity that groups several backup repositories into a single pool of storage, and allows to add or remove backup repositories as needed1. Hardened repository is a Linux-based backup repository with immutability enabled, which prevents backup files from being modified or deleted until the specified retention period expires2. The other options are either not immutable (option A and C, as NTFS and XFS do not support immutability) or not scalable (option B, as ReFS does not support adding or removing extents in a Scale-out Backup Repository3). References: Scale-out Backup Repository, Hardened Repository, ReFS Limitations.