At which layer of the OSI model is RSTP used to prevent loops?

At which layer of the OSI model is RSTP used to prevent loops?
A . physical
B . data link
C . network
D . transport

Answer: B

Explanation:

RSTP and STP operate on switches and are based on the exchange of Bridge Protocol Data Units (BPDUs) between switches. One of the most important fields in BPDUs is the Bridge Priority in which the MAC address is used to elect the Root Bridge , RSTP operates at Layer 2. http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml