Which option is used as a loop prevention mechanism to support MPLS VPN customers with multihomed sites?

Which option is used as a loop prevention mechanism to support MPLS VPN customers with
multihomed sites?

A.
BGP down bit

B.
sham links

C.
AS override

D.
SOO extended BGP community

E.
allow as-in

Explanation:
http://www.cisco.com/en/US/docs/ios/12_4t/12_4t11/htbgpsoo.html
Site of Origin BGP Community Attribute
The site-of-origin (SoO) extended community is a BGP extended community attribute that is used
to identify routes that have originated from a site so that the readvertisement of that prefix back to
the source site can be prevented. The SoO extended community uniquely identifies the site from
which a router has learned a route. BGP can use the SoO value associated with a route to prevent
routing loops.