Which high-availability mechanism is a detection protocol that is enabled at the interface and at the routing protocol levels?

Which high-availability mechanism is a detection protocol that is enabled at the interface and at the
routing protocol levels?

A.
NSF

B.
SSO

C.
NSR

D.
BFD

E.
SDR

Explanation:
http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fs_bfd.html