VLAN is a most useful technology, which is often used in different network environments. It is important for you to have a real understanding of the changes brought by VLAN. Refer to the following statements about VLAN, which two are correct?

A.
VLANs increase the size of collision domains.

B.
VLANs allow logical grouping of users by function.

C.
VLANs simplify switch administration.

D.
VLANs enhance network security.

Explanation:
Refer to the divided VLANs. The number of collision domains does not change because each switch port is a collision domain. So A is wrong.
VLANs do not simplify switch administration. On the contrary, many configurations are required. VTP can simplify theses configurations. C is wrong.
VLANs allow logical grouping of users by function. Communication cannot be achieved between VLANs without the layer 3 device, which restricts the mutual access and improves the network security.