What is the best description of the HA4 Keep-alive Threshold (ms)?

What is the best description of the HA4 Keep-alive Threshold (ms)?
A . the time that a passive or active-secondary firewall will wait before taking over as the active or active-primary firewall
B . the timeframe that the local firewall waits before going to Active state when another cluster member is preventing the cluster from fully synchronizing
C . the maximum interval between hello packets that are sent to verify that the HA functionality on the other firewall is operational
D . the timeframe within which the firewall must receive keepalives from a cluster member to know that the cluster member is functional

Answer: D