In context to VM migration, what feature ensures it is done without downtime during a cluster upgrade?

The feature that enables virtual machine migration without downtime during a cluster upgrade is vSphere vMotion. vMotion allows for the live migration of running virtual machines from one physical host to another without disconnecting the client or shutting down the virtual machine. This capability is particularly valuable during maintenance activities, such as cluster upgrades, as it ensures that workloads can continue to run while the underlying infrastructure is being updated.

With vMotion, not only is the virtual machine's state transferred seamlessly, but it also ensures that there is no interruption in service, which is critical in environments where high availability is necessary. This allows administrators to perform maintenance tasks on the underlying hardware while maintaining service continuity for users.

In the context of the other options: vSphere Replication is primarily used for disaster recovery purposes, ensuring data is replicated to a secondary site rather than facilitating live migration. Storage Dead Paths Management deals with identifying and managing storage paths that are not in use, which does not pertain to live VM migration. Distributed Resource Scheduler optimizes resource allocation across hosts in a cluster based on current workloads but does not facilitate migration without downtime.