Nutanix documentation for Performing Failover in PD-based DR includes the exact error message quoted in the question: “Protection domain is already active on source site ... Please trigger migrate from source site to activate the protection domain.” Nutanix shows this message when the protection domain is still active on the source cluster and the proper action is a planned migrate from the source, not an activate at the target. In other words, the source cluster is still reachable and authoritative, so trying to activate the protection domain on the other side is the wrong operation. That makes D the correct answer. ( Nutanix Portal )

This is a classic Nutanix DR distinction between planned migration and unplanned failover/activation. If the primary cluster is still available, Nutanix expects the administrator to use migrate from the source site. Activation at the destination is reserved for failover conditions when the source cannot safely coordinate the move. The error itself tells you the reason if you read it carefully: the protection domain is still active on the source site. Therefore the root cause is that the administrator is trying to activate the PD while the primary cluster is still reachable and still owns the active copy, which is exactly option D. ( Nutanix Portal )