Answer:
VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol that allows switches to exchange VLAN information. It simplifies the management of VLANs by allowing the configuration of VLANs on a single switch and then propagating that information to other switches in the network.

The correct answer is option 'A' - All switches are VTP servers by default. This means that when a switch is first powered on, it operates as a VTP server by default. Let's discuss this in more detail:

Explanation:
Default VTP Modes:
- VTP Server: A switch in server mode is responsible for creating, modifying, and deleting VLANs. It also shares this information with other switches in the VTP domain.
- VTP Client: A switch in client mode receives VLAN information from VTP servers and cannot create, modify, or delete VLANs. It relies on the VTP server for all VLAN configurations.
- VTP Transparent: A switch in transparent mode does not participate in VTP updates. It forwards VTP advertisements but does not synchronize or update its own VLAN database.

VTP Server by Default:
When a Cisco switch is first powered on, it operates in VTP server mode by default. This means that it can create, modify, and delete VLANs, and it advertises this information to other switches in the VTP domain. However, it is important to note that the default configuration of VTP on a Cisco switch is "VTP domain: null" and "VTP mode: server".

Other VTP Modes:
- VTP Client: A switch can be configured as a VTP client manually. In this mode, it receives VLAN information from VTP servers and cannot create, modify, or delete VLANs. It relies on the VTP server for all VLAN configurations.
- VTP Transparent: A switch can also be manually configured as a VTP transparent switch. In this mode, it does not participate in VTP updates. It forwards VTP advertisements but does not synchronize or update its own VLAN database.

Conclusion:
In summary, the default VTP mode for a Cisco switch is VTP server mode. However, it is important to configure the VTP domain name and mode appropriately based on the network requirements. It is recommended to always verify the VTP mode and domain name on each switch to ensure proper VLAN management and avoid any unintentional changes.