It is possible for a Cisco Catalyst switch to check if a Cisco IP Phone is directly attached to that port by:

As a network administrator, you issue the interface auto qos voip cisco-phone command on a port in an edge network. It is possible for a Cisco Catalyst switch to check if a Cisco IP Phone is directly attached to that port by:

A.
using DHCP snooping

B.
snooping the incoming 802.1Q VLAN tag

C.
using CDP

D.
snooping the CoS marking on the incoming frames