What should a solutions architect do to meet these requirements MOST cost-effectively?

Explanations:

Migrating the application layer to EC2 Spot Instances could lead to potential interruptions since Spot Instances can be terminated by AWS when capacity is needed, which is not suitable for a 24/7 application. Storing the data in Amazon S3 is also not ideal as S3 is an object storage service and may not provide the necessary database capabilities for the application.

While migrating to EC2 Reserved Instances would provide cost savings for a 24/7 application, using Amazon RDS On-Demand Instances for the database would not be cost-effective in the long term due to the continuously growing storage needs. Reserved Instances for RDS would be more suitable for predictable workloads.

Migrating the application layer to Amazon EC2 Reserved Instances ensures lower costs for a 24/7 application. Using Amazon Aurora Reserved Instances for the database layer also aligns with the growth of the database storage over time, providing a scalable and cost-effective solution. Aurora is designed for high availability and can efficiently handle growing data needs.

Although using EC2 On-Demand Instances provides flexibility, it is generally more expensive than Reserved Instances for a continuously running application. While migrating the data storage layer to Amazon RDS Reserved Instances is appropriate, the overall solution is not the most cost-effective compared to using EC2 Reserved Instances.