A network engineer wants to apply for new license key file. Which command should be used to obtain the switch serial number?

'show license serial-number' is not a valid NX-OS command on Cisco MDS switches; the licensing subsystem uses the host-id keyword, not a serial-number keyword.

The running configuration does not store a serial-number field, so piping 'show running-config' through 'include serial-number' returns no output and does not produce the host ID needed for licensing.

'show host-id' is not a recognized NX-OS command syntax; the correct command requires the 'license' keyword as in 'show license host-id'.

The 'show license host-id' command displays the unique host ID that Cisco's licensing portal uses to generate and cryptographically bind a license file to a specific MDS switch. This host ID, not the physical serial number, is the required input when submitting a license request.