The correct choice is EC2 On-Demand Instances. On-Demand pricing requires no upfront payment and no long term commitment and instances are not subject to AWS price-driven interruptions so it provides predictable costs and continuous availability for a 45 day pilot.

On-Demand billing is usage based so you pay only for the instance hours or seconds you consume and you can stop the instances at the end of the pilot to avoid further charges. Because AWS does not reclaim On-Demand instances for price reasons this option meets the requirement that the startup will not be interrupted.

Spot Instances are the least expensive option but they can be reclaimed by AWS with little notice so they do not satisfy a requirement to avoid interruptions during the pilot.

Reserved Instances require a one or three year commitment or an upfront payment and they are therefore unsuitable when the scenario specifies no long term commitment even though they provide cost savings for steady long term workloads.

Compute Savings Plans provide billing discounts in exchange for a one or three year commitment to a consistent spend rate and they do not change instance interruption behavior or provide capacity guarantees that would remove the risk of interruptions.

Full AWS Practitioner Certification Question