A reporting web application runs on EC2 instances only weekly and at month end and can be stopped when idle. Which EC2 pricing model is the most cost-effective for this use case?

On-Demand Instances are the most cost-effective for this scenario because you pay only for the compute capacity you use, with no long-term commitments. Since the application runs only weekly and at month-end, and can be stopped when idle, On-Demand instances allow you to start and stop them as needed, minimizing costs. Standard Reserved Instances and Convertible Reserved Instances require a commitment (typically one or three years) and are designed for steady-state workloads, making them less suitable for an application that runs intermittently. On-Demand Capacity Reservations reserve capacity but still charge at the On-Demand rate, and while they guarantee availability, they don't offer a cost advantage for an application that is frequently stopped.