Which solution is needed to enable presence and extension mobility to branch office phones during a WAN failure?

Which solution is needed to enable presence and extension mobility to branch office phones during
a WAN failure?

A.
SRST with MGCP fallback

B.
SRST without MGCP fallback

C.
Cisco Unified Communications Manager Express in SRST mode

D.
SRST with VoIP dial peers to Cisco Unified Communications Manager Express