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A failover cluster is designed to provide high availability and reliability for applications and services. It consists of multiple systems, often referred to as nodes, that work together to support a service. In the event that one node fails, another can take over the responsibilities, ensuring that there is minimal downtime and that services remain available to users. This redundancy is key to maintaining service continuity and is a fundamental characteristic of failover clusters.

Each node in the cluster can monitor the health of the other nodes and respond to failures by transferring workload and resources to a functioning node. Since the purpose of a failover cluster is to maintain service availability, having multiple systems is crucial; this distributed approach helps achieve resilience against hardware or software failures.

The other options do not reflect the nature of a failover cluster. A single backup server would not provide the redundancy required, a fixed server implies lack of flexibility and adaptability, and a storage unit alone does not accommodate the operational requirements of service handling and high availability that a failover cluster is designed to fulfill. Therefore, the correct answer illustrates the essential characteristic of failover clusters being comprised of multiple systems working collaboratively to ensure continuous service.