Refer to the exhibit.

Spanning tree protocol is running on all three switches. The switches are configured so that Link
A is the active link, and Link B is the standby link. There is a problem occurring where Switch B
starts forwarding on Link B causing a routing loop.
What is the likely cause of the problem?

A.
PortFast is not enabled.

B.
There is a port duplex mismatch.

C.
MISTP is enabled without RSTP.

D.
A single instance of STP is enabled instead of PVST.