What is the function of the command switchport trunk native vlan 999 on a Cisco Catalyst switch?

What is the function of the command switchport trunk native vlan 999 on a Cisco Catalyst switch?

A.
It designates VLAN 999 for untagged traffic.

B.
It blocks VLAN 999 traffic from passing on the trunk

C.
It creates a VLAN 999 interface.

D.
It designates VLAN 999 as the default for all unknown tagged traffic.

Explanation:
Configuring the Native VLAN for Untagged TrafficA trunk port configured with 802.1Q tagging can

receive both taggedand untagged traffic. By default, the switch forwards untagged trafficin the
native VLAN configured for the port. The native VLAN is VLAN 1by default.