Final answer:
Azure Disk Storage is the storage service that must be used to store the unmanaged data disks of a virtual machine in Azure.
Step-by-step explanation:
The storage service that must be used to store the unmanaged data disks of a virtual machine in Azure is Azure Disk Storage.
Azure Disk Storage is a managed disk service in Azure that allows you to store and manage the data disks for your VMs.
With Azure Disk Storage, you can choose between premium and standard disks, depending on your performance and cost requirements.