Which instance purchasing option will meet this requirement MOST cost-effectively?

Explanations:

On-Demand Instances are billed by the second (for Linux instances) or by the hour (for Windows instances) with no long-term commitments. For short-term, predictable workloads like this 12-hour experiment, On-Demand is the most cost-effective option. You only pay for the time the instance is running.

Reserved Instances require a 1-year or 3-year commitment. For a workload that only runs for 12 hours at a time, paying for a Reserved Instance is extremely cost-inefficient.

Spot Instances are cheaper than On-Demand but can be interrupted with little notice. While the workload is short, the requirement tocontinuously runsuggests that interruptions are undesirable, making Spot Instances unsuitable.

Dedicated Instances are physical servers dedicated to a single customer. They are the most expensive option and not necessary for this type of workload.