The correct option is Reliability. Deploying an Amazon Aurora MySQL cluster across multiple Availability Zones with automatic failover is an availability and fault tolerance design that keeps the application running during an AZ disruption.

By placing instances and storage across AZs and using automatic failover, Aurora improves fault tolerance and enables rapid recovery so services remain available when an AZ fails. These capabilities map to the Reliability pillar which emphasizes recovering from failures, meeting availability targets, and automating restoration of service.

Security focuses on protecting data and systems with controls such as identity and access management, encryption, and monitoring. Multi-AZ failover does not primarily address those protection controls so it is not the best match.

Cost optimization concentrates on controlling and reducing spend through right sizing, pricing choices, and eliminating waste. Adding Multi-AZ replicas typically increases cost to gain resilience so it does not align with cost optimization.

Performance efficiency is about selecting the right resources and scaling to meet performance requirements. Multi-AZ deployments improve availability and fault tolerance more than they directly improve raw performance so they align less with performance efficiency.

Full AWS Practitioner Certification Question