Which EC2 instance purchasing option will meet these requirements MOST cost-effectively?

Explanations:

Standard Reserved Instances offer a significant discount for a one-year commitment and are ideal for predictable workloads that run constantly, such as a database server.

Convertible Reserved Instances allow flexibility to change instance types, but they are generally more expensive than Standard Reserved Instances for a specific one-year commitment.

On-Demand Instances are more expensive for continuous use over a long period, as they are billed hourly without any long-term discounts.

Spot Instances are the least expensive option, but they can be interrupted by AWS, making them unsuitable for a database server that needs to run constantly.