According to the VMware Cloud Foundation documentation1, an external SFTP server is recommended for storing backups of SDDC Manager and NSX-T Manager for the following reasons:

It provides better protection against failures because it decouples NSX-T backups from SDDC Manager.

It is a prerequisite for restoring SDDC Manager file-based backups.

Therefore, statements B and D are correct and should be shared with the customer.

References:

1: Backup and Restore of VMware Cloud Foundation

https://docs.vmware.com/en/VMware-Cloud-Foundation/5.0/vcf-admin/GUID-F8634D37-FA26-40DF-A135-62D0265DA4FA.html

By default, NSX Manager file-based backups are taken and stored on an SFTP server that is built into the SDDC Manager appliance. It is recommended that you configure an external SFTP server as a backup location for the following reasons: An external SFTP server is a prerequisite for restoring SDDC Manager file-based backups. Using an external SFTP server provides better protection against failures because it decouples NSX backups from SDDC Manager.