The answer incorporating an auto-scaling group, an Elastic Load Balancer (ELB), and Web Application Firewall (WAF) correctly brings together elasticity, load distribution, and web traffic security, meeting the requirements for both scalability and protection against web exploits. An auto-scaling group automatically adjusts the number of instances according to demand. An ELB efficiently distributes incoming application traffic across multiple targets, and WAF helps protect the app from common web exploits. Though the option with just an auto-scaling group and ELB ensures scalability, it lacks the security element against web exploits. A solution with multiple Availability Zones does contribute to high availability but does not directly address scalability and web security. Deploying instances in a public subnet only considers the network configuration and does not address the multi-tier application's scale and security needs.