Which feature is used to verify that the VPN tunnel is up even if user traffic is not passing through it?

A site-to-site VPN is configured between the main office and a remote office. An administrator wants to keep track of the VPN tunnel.

Which feature is used to verify that the VPN tunnel is up even if user traffic is not passing through it?

A.
Dead peer detection sending ICMP packets

B.
VPN monitoring sending ICMP packets

C.
VPN monitoring sending UDP packets

D.
Dead peer detection sending UDP packets