Extending a VM's disk size

These instructions worked well

When you extend a VM disk, you first change its disk size properties from the host. This doesn't allocate the new diskspace, it only stakes a claim to it. After changing the disk size value, start the VM and open DISKMGMT.MSC to see the new partition. If there is a "Recovery" partition against the primary partition and the new one, follow the steps below. (other wise, just extending the volume).

If there is a "Health (Recovery Partition)," the newly added volume of the disk will be after that recovery partition. In the image below, the new, unallocated disk volume appeared after the recovery partition.

Extending

Recovery partitions can be deleted. But you need to use the override option to do so.

List the partitions to show their numbers.
Select the recovery partition number.
Then delete the recovery partition.

delete partition override

That should make the existing volume and the new volume contiguous. Right-click the new volume to extend it.

The reason you cannot extend the drive in Disk Management is that Windows can only extend a partition into contiguous (touching) unallocated space directly to its right. The 748MB Recovery Partition is acting as a "wall" between your C: drive and the new 100GB of space.

Since this is a VM, you have two main ways to fix this. Before starting, take a Snapshot of your VM so you can revert if something goes wrong.

You can't extend drive C until you remove the recovery partition.

Option 1: Delete the Recovery Partition (Easiest/Recommended for VMs)

Most VMs do not actually need the built-in Windows Recovery partition because you likely use snapshots or backups at the hypervisor level. Deleting it is the fastest way to merge the space.

Right-click the Start button and select Command Prompt (Admin) or PowerShell (Admin).
Type diskpart and press Enter.
Type list disk (Identify your main disk, usually Disk 0).
Type select disk 0.
Type list partition.
Identify the Recovery partition (it should be roughly 748MB). Let's assume it is Partition 4.
Type select partition 4 (Replace "4" with your actual recovery partition number).
Type delete partition override.
Now, go back to Disk Management. The 100GB and the 748MB should now be combined into one "Unallocated" block directly next to your C: drive.
Right-click your C: drive and select Extend Volume.

Summary Recommendation

If this is a standard server or workstation VM, Option 1 (deleting the partition via Diskpart) is the standard industry practice. It takes 30 seconds and causes no issues with Windows booting or operation. Just ensure you have a fresh Snapshot before you run the delete partition override command.