A company must run a web server on Amazon EC2 instances for at least one year, and the server cannot be interrupted. Which EC2 purchasing option is the most cost-effective for this scenario?

Partial Upfront Reserved Instances are the most cost-effective because they offer a significant discount compared to On-Demand pricing in exchange for a commitment to use the instance for a specific term (one or three years) and paying a portion of the cost upfront. This aligns perfectly with the requirement for a non-interruptible web server running for at least one year. On-Demand Instances are more expensive as they offer no commitment or upfront payment. Spot Instances are the cheapest but are unsuitable because they can be interrupted by AWS with two minutes' notice, which violates the requirement for an uninterrupted server. No Upfront Reserved Instances offer a discount for a commitment but typically provide a smaller discount than Partial Upfront options, making them less cost-effective for this scenario.