Azure Update Management can be enabled for virtual machines regardless of their operating system, as long as they are supported by Azure and meet the requirements for the Update Management service. This service supports various operating systems including Windows Server and Linux distributions.
Based on the operating systems listed:
VM1: Windows Server 2012
VM2: Windows Server 2012 R2
VM3: Windows Server 2016
VM4: Ubuntu Server 18.04 LTS
All of these operating systems are supported by Azure Update Management. Moreover, the state of the virtual machine (running or stopped) does not affect whether Update Management can be enabled; it only affects whether updates can be applied directly. Update Management can be enabled on a stopped VM, but updates can only be applied when it is running.
Therefore, the correct answer is:
D. VM1, VM2, VM3, and VM4
All these VMs can have Update Management enabled to manage and apply updates across the different operating systems.