Which of the following is NOT a VTP mode?

In the context of VLAN Trunking Protocol (VTP), the modes that define how a switch interacts with VLAN resources are Server, Client, and Transparent.

The Server mode allows a switch to create, modify, and delete VLANs for the entire VTP domain. Clients can participate in the VTP domain by receiving information about VLANs but cannot make any changes to the VLAN configuration. The Transparent mode allows a switch to pass VTP advertisements to other switches, but it does not participate in any VTP domain itself; it does not send or receive VLAN information.

The choice that does not belong to these established VTP modes is "Isolated." This is not a recognized mode within VTP and does not function within the framework of how VTP is designed to manage VLANs across network devices. Understanding these modes is crucial for network design and management as it helps to maintain consistent VLAN configurations and prevent misconfigurations across switches in the same network.