Which command can be issued without interfering with the operation of loop guard?

A.
Switch(config-if)#spanning-tree guard root

B.
Switch(config-if)#spanning-tree portfast

C.
Switch(config-if)#switchport mode trunk

D.
Switch(config-if)#switchport mode access

Explanation:
The spanning-tree guard root cannot be enabled together with loop guard. The loop guard feature
is supposed to be used on the port receiving BPDU and guard root will shutdown the port as soon
as the first BPDU comes to that port.
Configuring portfast on the port connected to the other switch can create temporal loop.
Configuring access mode on the port can filter BPDU from other VLANs from coming to the port
and force loop guard feature to put this port into error-disabled state.

So the only command that can be issued without interfering with the operation of loop guard is
“switchport mode trunk”.
Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml