The best answer is C. Operate in fail-secure mode.

If the goal is to prevent unauthorized entry during a power outage, the doors should remain locked when power fails. That is exactly what fail-secure operation is intended to do. Alarming and monitoring may detect an issue, but they do not themselves prevent entry. Physical key backup may help authorized access, but it does not address the primary requirement of staying secure during loss of power.

So the strongest answer is fail-secure, because it directly preserves physical access control during an outage.

References (Official ISACA):

ISACA physical security and access-control terminology resources support layered protection of facilities.