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.

If another switch advertises a superior BPDU, or one with a better Bridge ID, on a port where root guard is enabled, the local switch will not allow the new switch to become the root. As long as the superior BPDUs are being received on the port, the port will be kept in the root-inconsistent STP state.

Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96b.shtml