Exhibit.

The appearance of a serial number in the Native VLAN column for port23 suggests that the switch connected to this port is identified uniquely in the network. Given the options provided:

A standalone switch with the shown serial number is connected on port23 (Option C): This is the most plausible explanation. The FortiSwitch configuration interface is displaying the serial number of a standalone switch that is directly connected to port23. This kind of display helps in identifying and managing individual devices in a network setup, especially in environments with multiple switches.