A company runs a web application on multiple Amazon EC2 instances in an Auto Scaling group that scales based on demand. The company wants to maximize cost savings without making long-term commitments. Which EC2 purchasing option should a solutions architect recommend?

A mix of On-Demand Instances and Spot Instances is the most cost-effective solution for a web application with fluctuating demand that wants to maximize savings without long-term commitments. On-Demand Instances provide a baseline of stable, uninterrupted capacity for critical workloads, ensuring the application remains available. Spot Instances offer significant cost savings (up to 90% off On-Demand prices) for fault-tolerant workloads, which is suitable for web application servers that can be interrupted and replaced by the Auto Scaling group. Dedicated Instances are more expensive and offer isolation, which isn't the primary concern here. On-Demand Instances only would be reliable but would not maximize cost savings. A mix of On-Demand and Reserved Instances would provide cost savings for predictable, long-term usage, but the question specifies "without making long-term commitments.