Which of the following specifications is a companion to the IEEE 802.1w Rapid Spanning
Tree Protocol (RSTP) algorithm, and warrants the use multiple spanning-trees?

A.
IEEE 802.1s (MST)

B.
IEEE 802.1Q (CST)

C.
Cisco PVST+

D.
IEEE 802.1d (STP)

E.
None of the other alternatives apply

Explanation:
MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP).
MST extends the IEEE 802.1w rapid spanning tree (RST) algorithm to multiple spanning
trees. This extension provides both rapid convergence and load balancing in a VLAN
environment. MST converges faster than PVST+. MST is backward compatible with 802.1D
STP, 802.1w (rapid spanning tree protocol [RSTP]), and the Cisco PVST+ architecture.
MST allows you to build multiple spanning trees over trunks. You can group and associate
VLANs to spanning tree instances. Each instance can have a topology independent of other
spanning tree instances. This new architecture provides multiple forwarding paths for data
traffic and enables load balancing. Network fault tolerance is improved because a failure in
one instance (forwarding path) does not affect other instances (forwarding paths).
In large networks, you can more easily administer the network and use redundant paths by
locating different VLAN and spanning tree instance assignments in different parts of the
network. A spanning tree instance can exist only on bridges that have compatible VLAN
instance assignments. You must configure a set of bridges with the same MST configuration
information, which allows them to participate in a specific set of spanning tree instances.
Interconnected bridges that have the same MST configuration are referred to as an MST
region.
Reference:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_cha
pter09186a008007e71a.html#wp1082480