The correct answer is D because the application is written entirely in JavaScript and runs in users’ web browsers , which means the workload is essentially a static web application . Static assets such as HTML, JavaScript, CSS, and related files are best hosted on Amazon S3 , which provides highly durable and low-cost object storage. Putting Amazon CloudFront in front of the S3 bucket allows the application to be delivered globally through edge locations, which reduces latency for users in many countries while also lowering the load on the origin.

This design is more cost-effective than continuing to serve the application from EC2 instances behind an ALB because it eliminates most of the compute and load balancing cost for static content delivery. CloudFront caches the content close to users and can improve performance worldwide without the complexity of deploying full application stacks in multiple Regions.

Option A is less cost-effective because it still depends on the ALB and EC2 instances as the origin for content that is static. Also, CloudFront requires an ACM certificate in us-east-1 for custom domain names, so reusing the existing certificate from the ALB is not the right assumption. Option B introduces significant multi-Region infrastructure cost and management overhead. Option C is unnecessarily complex because multiple S3 buckets in multiple Regions are not required when CloudFront can cache globally from a single origin.

AWS best practices for static web applications recommend Amazon S3 for storage and Amazon CloudFront for global distribution . This approach provides strong performance, simplicity, and cost optimization.