Which instance purchasing option will meet these requirements MOST cost-effectively?

Explanations:

On-Demand Instances allow you to pay for compute capacity by the hour or second with no long-term commitments. They can be launched and terminated at any time without interruptions, making them suitable for applications that require constant availability.

Spot Instances are cost-effective but can be interrupted with little notice when the spot price exceeds your bid price. This option does not meet the requirement for non-interruption.

Dedicated Hosts are physical servers dedicated to your use, but they require a longer-term commitment and upfront payment. They are not the most cost-effective option for a non-committed purchasing model.

Reserved Instances provide a discounted rate in exchange for a commitment to use a specific instance type for a one- or three-year term. This option does not meet the requirement for no long-term commitment or upfront payment.