Which type of attack can be prevented by setting the native VLAN to an unused VLAN?

A.
VLAN-hopping attacks

B.
CAM-table overflow

C.
denial-of-service attacks

D.
MAC-address spoofing

Explanation:
Another type of VLAN attack is a double-tagging(or double-encapsulated) VLAN hopping attack.
This type of attack takes advantage of the way that hardware on most switches operates. Most
switches perform only one level of 802.1Q de-encapsulation, which allows an attacker to embed a
hidden 802.1Q tag inside the frame. This tag allows the frame to be forwarded to a VLAN that the
original 802.1Q tag did not specify.
The best approach to mitigating double-tagging attacks is to ensure that the native VLAN of the
trunk ports is different from the VLAN of any user ports. In fact, it is considered a security best
practice to use a fixed VLAN that is distinct from all user VLANs in the switched network as the
native VLAN for all 802.1Q trunks.
Reference: http://www.ciscopress.com/articles/article.asp?p=2181837&seqNum=10