What is the MOST cost-effective instance purchasing option that meets these requirements?

Explanations:

Spot Instances are often cheaper but can be interrupted, making them unsuitable for an application that must always be available.

Savings Plans provide a flexible pricing model that offers significant savings over On-Demand pricing while ensuring the application remains available continuously.

Dedicated Hosts are more expensive and intended for compliance and licensing needs, which is unnecessary for a standard ecommerce application.

On-Demand Instances are flexible but are typically more expensive than Savings Plans over a long term, making them less cost-effective for continuous use over 12 months.