What is the MOST cost-effective instance purchasing option to meet these requirements?

Explanations:

Compute Savings Plans require a consistent workload and are beneficial for continuous usage. They are not cost-effective for sporadic, low-hour usage like 3 hours per week.

On-Demand Instances offer flexibility for short, infrequent workloads, such as running for a few hours each week, without any long-term commitment or interruptions.

Convertible Reserved Instances require a commitment period (1 or 3 years) and are best for workloads with more consistent and predictable usage.

Spot Instances are not suitable because they can be interrupted at any time, which does not meet the requirement of non-interruptible workloads.