An e-learning provider runs an application for only 2 months each year on Amazon EC2 and cannot tolerate any downtime during those months. Which EC2 purchasing option is the most cost-effective for this scenario?

On-Demand Instances are the most cost-effective for this scenario because they offer flexibility without a long-term commitment. You pay for compute capacity by the hour or second, with no upfront costs or minimum contracts. Since the application runs for only 2 months annually, the cost savings from Reserved Instances (which require a 1- or 3-year commitment) would not be realized, and you'd pay for unused capacity. Dedicated Hosts are expensive and provide physical server isolation, which is unnecessary here. Spot Instances are the cheapest but are unsuitable due to the "cannot tolerate any downtime" requirement, as they can be interrupted with short notice.