Which feature of VMware vCenter Server allows for seamless migration of virtual machines without downtime?

vMotion is the correct feature of VMware vCenter Server that facilitates the seamless migration of virtual machines (VMs) without downtime. This technology enables administrators to transfer a running VM from one physical server to another while it continues to operate. vMotion works by leveraging shared storage and the ability to move the VM's memory and CPU state over the network, allowing for the live migration without interruption of service.

This functionality is crucial for maintaining availability and performance during maintenance tasks or server load balancing, as it permits the redistribution of VMs across hosts in a cluster to optimize resource usage. As a result, vMotion is an essential tool for ensuring minimal impact on end users during VM management tasks.

In contrast, other options focus on different aspects of VMware's virtualization capabilities; for example, vSAN is a storage solution, VMware High Availability (HA) provides automatic VM restarts in case of host failures, and DRS deals with resource allocation and load balancing but does not migrate VMs without downtime.