A company runs EC2 instances in a research lab for 3 hours each week and the instances must not be interrupted. Which EC2 purchasing option is the most economical for these requirements?

On-Demand Instances are the most economical choice here because they offer pay-as-you-go pricing without upfront commitments, and you only pay for the compute capacity you use by the second (after the first minute). Since the instances are needed for a short, fixed duration of 3 hours per week and cannot be interrupted, On-Demand provides the flexibility and guaranteed availability required without the long-term commitment of Reserved Instances. Compute Savings Plans offer lower prices than On-Demand but require a 1-year or 3-year commitment, which is not suitable for only 3 hours of usage per week. Convertible Reserved Instances also require a 1-year or 3-year commitment. Spot Instances are the most cost-effective for flexible workloads but are unsuitable because they can be interrupted with a two-minute notification, which violates the requirement that the instances "must not be interrupted.