For a reporting application that runs only periodically, On-Demand Instances are the most cost-effective choice because they allow the company to pay only for the compute capacity used, without long-term commitments. Reserved Instances are less flexible due to the need for upfront payment or long-term contracts, which would not be cost-effective given the application's intermittent usage. On-Demand Capacity Reservations would also be more costly, as they hold capacity regardless of usage.