The correct answer is D, Reliability. The AWS Well-Architected Framework's Reliability pillar focuses on the ability of a system to recover from infrastructure or service disruptions. The use of Amazon EC2 Auto Scaling and an Application Load Balancer to automatically recover unhealthy instances aligns with the principles of this pillar. Horizontal scaling, as implied by the question, is a method to increase aggregate workload availability and maintain service continuity, which is a key aspect of ensuring reliability.