Which of the following connects a server and a SAN to improve performance?

The choice of a host bus adapter (HBA) is crucial for connecting a server to a storage area network (SAN) and enhancing performance. An HBA is a dedicated hardware component that connects the server's internal bus to the SAN, typically using SCSI or Fibre Channel protocols. By offloading storage traffic from the server's CPU and handling data transfer more efficiently, an HBA can significantly reduce latency and increase throughput, which is essential for applications that require high-speed data access.

In contrast, a network interface card (NIC) primarily facilitates network connections for data transmission over a local area network (LAN), making it less suited for the specialized requirements of SANs. Ethernet is also used for networking but does not inherently provide the same level of performance and features necessary for SAN connectivity as an HBA does. SCSI is a protocol for connecting and transferring data from storage devices but does not address the physical connectivity and performance enhancement aspects that an HBA provides in a SAN environment. Thus, the host bus adapter stands out as the optimal choice for connecting a server to a SAN to improve performance.