How can these requirements be met with the LEAST amount of effort?

Explanations:

AWS Elastic Beanstalk is a Platform as a Service (PaaS) that can automatically scale the Java web service and handle infrastructure management. By using Elastic Beanstalk with Auto Scaling, the application can easily scale to meet the expected 10x traffic increase without requiring major changes. This option meets the business requirements with minimal effort.

AWS SMS (Server Migration Service) is used for migrating existing on-premises servers to AWS. However, this does not offer automatic scaling, and would require managing the server and scaling configurations manually. This approach is not optimal for handling a significant traffic increase with minimal effort.

Migrating to Amazon ECS (Elastic Container Service) would require significant changes to the application, including creating a Docker image and modifying application code to work within ECS containers. This adds more complexity than using Elastic Beanstalk, which already supports automatic scaling with minimal effort.

This option involves creating a new architecture with API Gateway and AWS Lambda. While Lambda offers scalability, rewriting the application to use API Gateway and Lambda involves significant changes to the application code, which is not the least effort approach. Also, Lambda may not be ideal for running a full Java-based web service.