What is the function of the command switchport trunk native vlan 999 on a trunk port?

What is the function of the command switchport trunk native vlan 999 on a trunk port?

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