Which specification provides a standard definition of Ethernet VLANs?

The correct answer, which is B, refers to the IEEE 802.1Q specification. This standard specifically defines how Virtual Local Area Networks (VLANs) operate over Ethernet networks. VLANs allow networks to be segmented logically based on factors other than physical location, improving network management and efficiency.

The 802.1Q standard introduces a method for tagging Ethernet frames with VLAN identifiers, which helps in distinguishing traffic belonging to different VLANs across the same physical network infrastructure. This is essential for creating isolated network segments that can enhance security and reduce broadcast traffic.

In contrast, the other specifications mentioned do not pertain to VLANs. For instance, 802.11 primarily deals with wireless networking standards, 802.3 covers the standards for Ethernet physical layer and media access control, and 802.15 is related to wireless personal area networks. Therefore, 802.1Q is the standard that specifically provides the framework for VLAN definitions and operations within Ethernet networks.