What do cluster environments use to identify node failures?

A.
SNMP community strings

B.
heartbeats

C.
OSPF link state advertisements

D.
network interface driver diagnostics

Explanation:

What Happens when a Node Times Out
Each node sends a heartbeat message to the cluster coordinator every HEARTBEAT_INTERVAL number of microseconds (as specified in the cluster configuration file). The cluster coordinator looks for this message from each node, and if it does not receive it within NODE_TIMEOUT microseconds, the cluster is reformed minus the node no longer sending heartbeat messages.
On a node that is not the cluster coordinator, and on which a node timeout occurs (that is, no heartbeat message has arrived within NODE_TIMEOUT seconds), the following sequence of events occurs:
The node tries to reform the cluster.
If the node cannot get a quorum (if it cannot get the cluster lock) then
The node halts (system reset).
References: http://docstore.mik.ua/manuals/hp-ux/en/B3936-90122/ch03s07.html