Which specification provides a standard definition of Ethernet VLANs?

The correct choice is associated with the specification that defines the standards for Virtual Local Area Networks (VLANs) in Ethernet networks. The specification outlines how VLANs operate, their tagging method, and how they encapsulate Ethernet frames to maintain data integrity across various network segments.

Specifically, this standard introduces a method for tagging Ethernet frames with VLAN information, enabling the separation of different traffic types within the same physical network infrastructure. This tagging process allows for the identification of the VLAN to which a particular frame belongs, facilitating better network management, security, and traffic segmentation.

The other specifications mentioned do not focus on VLAN definitions. 802.3 pertains to Ethernet and physical layer specifications but does not detail VLANs. 802.1X relates to port-based network access control but is not concerned with VLAN tagging specifically. 802.11 pertains to wireless networking standards and does not cover Ethernet VLANs at all. Thus, 802.1Q uniquely provides the necessary framework for implementing and managing VLANs in an Ethernet context.