Which type of virtual disk only consumes disk space as needed rather than reserving it upfront?

Thin provisioning is the correct choice because it refers to a storage allocation technique in virtualized environments that allows for the efficient use of disk space. Instead of reserving a fixed amount of storage upfront, thin provisioning allocates space dynamically based on actual usage. This means that resources are only consumed as data is written to the disk, which can lead to significant savings in storage costs and more efficient overall disk usage.

With thin provisioning, administrators can provision a large amount of storage to virtual machines while only using the actual disk space required at any given time. This flexibility is particularly beneficial in environments with variable workloads or when deploying multiple virtual machines.

In contrast, other provisioning types like static and thick provisioning allocate a predetermined amount of disk space upfront, which can lead to inefficient use if the allocated space isn't fully utilized. Dynamic provisioning, while sometimes used interchangeably with thin provisioning, doesn't specify the same emphasis on the efficient, on-demand allocation featured in thin provisioning specifically.