Which feature helps maintain zero data loss when a VM on an ESXi server fails unexpectedly?

Fault Tolerance is designed specifically to prevent data loss during unexpected failures. It achieves this by creating a live shadow instance of a virtual machine (VM) that runs simultaneously with the primary VM. In the event of a failure, such as a hardware issue or a VM crash, the shadow instance immediately takes over processing without any interruption. This mechanism ensures that there is no data loss between the primary VM and the secondary instance, as both are continuously in sync.

The other features serve different purposes. vSphere High Availability minimizes downtime by automatically restarting VMs on other hosts in the cluster if a host fails, but during the restart process, there may be a loss of data that was in-flight. Storage vMotion allows for the live migration of a VM's storage from one datastore to another without downtime, but it does not prevent data loss in the case of an unexpected VM failure. vMotion similarly allows for the live migration of a VM from one host to another without downtime, but, like Storage vMotion, it is not geared toward preventing data loss in case of hardware failures. Thus, Fault Tolerance is the feature explicitly designed to maintain zero data loss during such failures.