The correct answer is C, Reliability. The AWS Well-Architected Framework is designed to help cloud architects build secure, high-performing, resilient, and efficient infrastructure for their applications and workloads. The Reliability pillar specifically focuses on the ability of a workload to perform its intended function correctly and consistently at the expected time. This includes ensuring that the system can handle failures, recover from them, and mitigate the impact of these failures on the end-users. The other options, Performance efficiency (A), Operational excellence (B), and Security (D), are also important pillars of the framework but do not directly address the question's focus on the ability to perform intended functions correctly and consistently.