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 creates a VLAN 999 interface.

B.
It designates VLAN 999 for untagged traffic.

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

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

Explanation:
Configuring the Native VLAN for Untagged Traffic
A trunk port configured with 802.1Q tagging can receive both tagged and untagged traffic. By
default, the switch forwards untagged traffic in the native VLAN configured for the port. The native
VLAN is VLAN 1 by default.