A data engineer wants to add Amazon EC2 instances to support increased web traffic during a promotion. Which service would be the MOST cost-effective solution?

Using AWS Auto Scaling is the most cost-effective solution for managing increased web traffic during a promotion by adding Amazon EC2 instances. AWS Auto Scaling allows you to automatically adjust the number of EC2 instances in your application based on the traffic load. When traffic increases, Auto Scaling spins up additional instances to handle the load, and when traffic decreases, it reduces the number of running instances. This dynamic scaling helps ensure that you only pay for the resources you need at any given time, optimizing costs while maintaining performance.

In contrast to the other options, AWS Elastic Beanstalk simplifies the deployment and management of applications but may not provide the same level of granularity in scaling solely based on traffic patterns, potentially resulting in either under or over-provisioning of resources. AWS Lambda is a serverless compute service that may not directly apply to scenarios requiring traditional instances—especially if the application architecture necessitates stateful components or specific server configurations. AWS CodeDeploy is focused on automating code deployments to EC2 instances or on-premises servers; while useful for managing application deployments, it does not inherently address the scaling needs driven by varying web traffic.