The word "resiliency" refers to
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To test resiliency, you should verify how the end-to-end workload performs under intermittent failure conditions.
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Under this:-
HA - VMs in different zones to prevent app going down when one zone goes down
DR - Entire app infra moves to another region with minimum downtime
Testing resiliency assumes a degree of HA or DR or both.
C is the most proper answer in this case, as it will test resiliency (of everything including authentication) by simulating a downtime (shutting down all VMs in one zone).