What is the default operation mode of DTP?

Dynamic Trunking Protocol (DTP) is used to negotiate trunking between switches dynamically. The default operation mode of DTP is dynamic auto. In this mode, a switch will actively listen for DTP frames from neighboring switches. If a neighboring switch is configured to be in a mode that allows trunking, such as dynamic desirable or trunk mode, the switch in dynamic auto will form a trunk link. If there is no neighbor that wishes to establish a trunk, the switch will just act as an access port.

The dynamic desirable mode, while capable of actively asking to form a trunk, is not the default and requires more aggressive negotiation. The option that states no trunk formed describes a scenario that can occur but does not represent the default operation of DTP. The access mode is typically used for ports connected to end devices and does not facilitate trunking. Hence, dynamic auto is the correct answer, as it represents the default and passive approach towards forming trunk links with connected switches.