The correct answer is 'Reliability'. The Reliability pillar ensures that a system can recover from infrastructure or service disruptions, dynamically acquire resources to meet demand, and mitigate disruptions such as network issues. Focusing on Reliability will help the organization ensure that their application can withstand component failures and scale appropriately with demand. 'Operational Excellence' is about running and monitoring systems, 'Security' focuses on protecting information and systems, and 'Cost Optimization' involves avoiding unnecessary costs – but none of these directly address system adaptability and recovery from failures as the 'Reliability' pillar does.