Which cloud storage technology is basically a virtual hard drive for instanced or VMs?

Volume storage, also called block storage, exposes raw storage blocks to an operating system exactly as a physical hard drive does, enabling a VM to format, partition, and mount it as local disk. Examples include Amazon EBS and Azure Managed Disks, which are attached directly to VM instances for OS volumes, databases, and stateful applications requiring low-latency disk access.

Platform refers to a cloud service category (PaaS), not a storage technology type.

Database storage is a managed service for structured data queries and transactions, not raw block-level disk storage that an OS can mount as a drive.

Application is a software or service category, not a recognized cloud storage type.

Object storage holds data as discrete objects accessed via HTTP APIs and cannot be mounted as a filesystem or virtual hard drive by a VM operating system.