A company runs a production application on a fleet of Amazon EC2 instances that read messages from an Amazon SQS queue and process them in parallel. Message volume is unpredictable with intermittent spikes. The application must continually process messages without downtime. Which solution meets these requirements MOST cost-effectively?

This solution is most cost-effective because Reserved Instances (RIs) provide significant cost savings for predictable, long-term workloads, which is ideal for the baseline capacity of the application. On-Demand Instances offer flexibility to scale up and down with unpredictable spikes in message volume without upfront commitment, ensuring no downtime. Using Spot Instances exclusively would be risky due to their interruptible nature, potentially causing downtime during message spikes. Using Reserved Instances exclusively for maximum capacity would be expensive and inefficient for unpredictable spikes, as RIs are best for consistent usage. Using RIs for baseline and Spot Instances for additional capacity is an option but carries the risk of interruptions for the additional capacity, which might impact message processing during spikes.