What enables Class of Service (CoS) by defining a 3-bit field in a VLAN tag?

The IEEE 802.1Q standard defines the format for VLAN tagging in Ethernet networks. Within this standard, it specifies a 3-bit field known as the Priority Code Point (PCP), which is part of the VLAN tag. This PCP field is crucial for implementing Class of Service (CoS), as it allows for prioritization of different types of network traffic, enabling network devices to handle packets based on their importance or required quality of service.

By using the 3-bit field, up to eight priority levels can be established, allowing network administrators to classify and manage traffic based on application requirements. This prioritization is essential for performance-sensitive applications, as it ensures that critical traffic receives the appropriate level of service, especially in congested network environments.

The other standards mentioned, such as IEEE 802.1X, 802.3, and 802.11, pertain to different aspects of networking, such as network access control, the physical and data link layers of Ethernet, and wireless networking, respectively. They do not provide the same functionality for defining Class of Service through VLAN tagging, which is why they are not the correct answer.