The ability to maintain data access in the event of the loss of a network link, host, or device is called:

Failover clustering groups multiple servers into a cluster so that when any single node, network link, or device fails, another cluster node automatically assumes the workload and maintains uninterrupted access to resources. This technology is specifically architected to eliminate single points of failure and preserve availability during component failures. It is the standard solution for high-availability server environments where access must be maintained despite hardware or network loss.

NAS balancing is not a recognized or standardized technology and does not describe any mechanism for maintaining access during hardware or link failures.

LUN replication copies logical unit data between storage systems for disaster recovery purposes but does not provide automatic failover to maintain live access during a host or link failure.

Load balancing distributes traffic across multiple resources to improve throughput and performance but does not inherently trigger automatic failover when a host or link becomes unavailable.