Which change will cause downtime for a virtual machine size increase from D4s v3 to D8s v3?

Increasing the size of a virtual machine, such as changing from D4s v3 to D8s v3, necessitates a downtime period for the virtual machine to shut down, resize, and restart. This process involves deallocating the VM, altering its size, and then starting it back up.

During the resizing operation, the virtual machine cannot process requests or handle workloads, which directly results in downtime. The Azure platform specifically requires the VM to be stopped and deallocated to change its size. Therefore, the choice related to changing the size of the virtual machine is indeed the correct answer as it involves operations that directly impact the availability of the VM.

The other options—enabling Desired State Configuration Management, adding a managed disk, and adding a Puppet Agent extension—typically do not require the virtual machine to be stopped for their implementation. These tasks can generally be performed while the VM is running, allowing it to remain operational and minimizing any potential downtime.