Switch R1 has been configured with the root guard feature. What statement is true if the
spanning tree enhancement Root Guard is enabled?

A.
If BPDUs are not received on a non-designated port, the port is moved into the STP
loop-inconsistent blocked state

B.
IF BPDUs are received on a PortFast enabled port, the port is disabled.

C.
If superior BPDUs are received on a designated port, the interface is placed into the
root-inconsistent blocked state.

D.
If inferior BPDUs are received on a root port, all blocked ports become alternate
paths to the root bride.

Explanation:
Root guard is configured on a per-port basis, and does not allow the port to become a STP
root port. This means that the port is always STP-designated. If there is a better BPDU
received on this port, root guard will put the port into root-inconsistent STP state, rather than
taking the BPDU into account and electing a new STP root. Root guard needs to be enabled
on all ports where the root bridge should not appear. In a way one can configure a perimeter
around part of network where STP root is allowed to be located.
Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96b.s
html