What type of network storage employs a virtualization technique allowing for efficient data allocation?

The correct choice, which is VHD (Virtual Hard Disk), employs a virtualization technique that allows for efficient data allocation. VHDs are a file format that represents a virtual hard disk drive. They can contain what would normally be found on a physical hard disk, such as the file system, partitions, and data. This enables multiple operating systems to run concurrently on a single physical machine, sharing resources in an efficient manner, as data can be dynamically allocated and resized according to needs.

VHDs are particularly useful in environments where virtual machines are deployed, as they provide the flexibility to allocate storage space as needed without being tied to the limitations of physical hardware.

In contrast, Network Attached Storage (NAS) serves as a dedicated file storage that allows multiple clients to access data storage over a network but does not inherently utilize virtualization techniques for efficient data allocation. RAID (Redundant Array of Independent Disks) focuses on optimizing performance and redundancy by interspersing data across multiple drives, rather than providing virtualization per se. Cloud storage is a service model that allows users to store data online, relying on data centers managed by third parties, but does not utilize a virtualization technique at the data allocation level like VHDs do.