Which solution will meet these requirements?

Explanations:

Amazon DynamoDB is a NoSQL database service that requires modifications to the application to adapt to its data model. Since the requirement is to avoid database modifications, this option is not suitable.

Amazon RDS for MySQL is a managed relational database service that supports MySQL, but it may not efficiently handle the sporadic usage patterns without incurring unnecessary costs, especially during periods of low demand. While it does not require modifications, it is not the most cost-effective option for varying loads.

MySQL-compatible Amazon Aurora Serverless automatically adjusts its capacity based on application demand, making it a cost-effective choice for applications with sporadic usage patterns. It requires no database modifications, and it can scale down during low usage periods, helping to control costs effectively.

MySQL deployed on Amazon EC2 in an Auto Scaling group would require more management overhead and could incur higher costs due to the need for provisioning EC2 instances. While it can scale, it may not be as efficient or cost-effective as a serverless option, especially with unpredictable workloads.