When talking to someone who works for the customer, they mention they are working on...

The best option to still satisfy the requirement for offsite, immutable or air gapped backups if the cloud provider selected for archive storage does not support immutability is to set up Scale-out Backup Repository capacity tier to a different cloud provider than the archive tier. Scale-out Backup Repository is a logical entity that groups several backup repositories into a single pool of storage, and allows to move backup files between performance and capacity tiers1. The capacity tier can be configured to use Amazon S3 or Microsoft Azure Blob Storage, which support Object Lock feature for immutability2. By using a different cloud provider for the capacity tier than the archive tier, the backup data can be stored in two separate locations, with one copy being immutable or air gapped. The other options are either not feasible (option A, as the scenario does not mention another site), not secure (option C, as the backup data can still be accessed or modified when the repository is online), or not offsite (option D, as the scenario requires a copy of backup data in a public cloud). References: Scale-out Backup Repository, Object Storage Immutability.