Refer to the exhibit.
An IP phone that is connected through a Cisco Catalyst 3750 Series Switch is failing to register with Cisco Unified Communications Manager. When the user presses the settings button on the phone, the Operational VLAN ID shows a blank entry.
What is the most likely cause for this issue?
A . The switch may not be supplying inline power.
B . The spanning tree portfast command needs to be removed.
C . The trunk encapsulation is missing. The trunk must be configured for dot1.
E . Cisco Discovery Protocol is disabled on the switch.
F . The Operational VLAN ID of the phone always shows as blank. The Admin. VLAN ID should be 110.
Answer: D