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

Which feature must be enabled to use vSphere Fault Tolerance?

Which vSphere feature protects against host hardware failures by restarting virtual machines (VMs) on hosts that are running in the cluster?

A vSphere administrator sees the alarm:
vSphere HA virtual machine failed to failover
This occurred for virtual machines (VMs) in a cluster with vSphere High Availability (HA) enabled. The guest operating system on the VMs did not report a power failure.
What is a possible cause of this event?

Which software-defined, object-based storage solution provides shared storage for vSphere clusters without using traditional external storage?