Which option must be configured on an interface to enable it to carry traffic tagged with both VLAN tags?

To enable an interface to carry traffic tagged with both VLAN tags, it is essential to configure flexible VLAN tagging. This configuration allows the interface to support multiple VLANs simultaneously by recognizing the VLAN tags present in Ethernet frames. Flexible VLAN tagging is particularly beneficial in scenarios where a frame could carry multiple tags, such as in the case of Q-in-Q tunneling, which is often used to encapsulate VLANs for service provider networks.

This approach provides greater flexibility and scalability in managing VLANs and is critical for environments where traffic from multiple VLANs may traverse the same path, ensuring proper segmentation and flow of data.

The other options do not fulfill this requirement as effectively. Access VLAN typically applies to access ports that only carry traffic from a single VLAN and cannot handle tagged traffic for multiple VLANs. VLAN Trunking Protocol is a Cisco proprietary protocol designed to manage VLAN information across trunk links, but it does not directly configure the handling of VLAN tags on an interface. Static VLAN tagging provides a method to configure VLANs on a switch port, but it lacks the dynamic capabilities and flexibility necessary for managing multiple tags per frame. Only flexible VLAN tagging is designed specifically to handle scenarios involving multiple VLAN tags efficiently.