Which protocol is commonly used for VLAN tagging in Ethernet frames?

The correct choice is the protocol that defines the standard method for VLAN tagging in Ethernet frames, which is IEEE 802.1Q. This standard allows for the inclusion of a VLAN tag in an Ethernet frame, enabling the separation of network traffic into different virtual LANs. By tagging frames with a VLAN identifier, 802.1Q facilitates proper routing of traffic across switches that are configured to recognize multiple VLANs. This capability is essential for maintaining network efficiency, security, and organization in environments where multiple VLANs are in use.

The other choices refer to different IEEE standards that serve other purposes. For instance, IEEE 802.3ad defines link aggregation, which allows multiple physical links to combine into a single logical link to increase bandwidth and redundancy. IEEE 802.3u pertains to Fast Ethernet standards, primarily focusing on data rates of 100 Mbps. IEEE 802.1p, while related to quality of service (QoS) and prioritization in Ethernet networks, does not specifically address VLAN tagging. Thus, IEEE 802.1Q is the definitive protocol for VLAN tagging within the context of Ethernet frames.