Choose the correct statement about undoing the failover:
A. Undoing the failover switches the replica to the Failback state
B. After the Undo Failover option is performed, the VM replica is used as the original VM
C. All the changes performed at the replica are transferred to the original VM
D. Undoing the failover switches the replica back to the primary VM, discarding all changes that were made to the replica while it was running
D is correct.
https://helpcenter.veeam.com/docs/backup/vsphere/failover_plan_undo.html?ver=110
You can undo failover for all VMs added to the failover plan at once. When you undo failover, you switch the workload back to original VMs and discard all changes that were made to VM replicas during failover.