Which EC2 instance purchasing option should a solutions architect recommend to meet these requirements?

Explanations:

Dedicated Instances provide physical servers for your instances but are not cost-effective for optimizing costs with fluctuating demand, as they require a long-term commitment and do not utilize pricing flexibility like On-Demand or Spot Instances.

On-Demand Instances allow for pay-as-you-go pricing, which is flexible, but they do not provide the potential for significant cost savings compared to Spot Instances, especially for variable workloads.

A mix of On-Demand Instances and Spot Instances allows for cost optimization. On-Demand Instances provide flexibility for baseline capacity, while Spot Instances can significantly reduce costs for non-critical workloads that can tolerate interruptions.

A mix of On-Demand Instances and Reserved Instances is not suitable for optimizing costs without a long-term commitment, as Reserved Instances require a commitment of 1 or 3 years, which contradicts the requirement for flexibility in cost savings.