A company runs a photo-sharing application. Users upload photos to an Amazon S3 bucket at a rate of about 150 photos per day. The company wants a solution that creates a thumbnail for each new photo and stores the thumbnail in a second S3 bucket. Which solution meets these requirements MOST cost-effectively?

The most cost-effective solution is to use S3 event notifications to trigger an AWS Lambda function. S3 event notifications are designed for real-time processing of S3 object changes, and Lambda functions are serverless, meaning you only pay for the compute time consumed when the function runs. With only 150 photos per day, this event-driven, serverless approach is highly efficient and scales automatically. The options involving Amazon EMR or EC2 instances are less cost-effective because they involve provisioning and maintaining long-running or always-on compute resources, even if they are largely idle. These services are better suited for continuous, heavy workloads, not infrequent, event-driven tasks. S3 Storage Lens is an analytics service for S3 usage and activity, not a mechanism for triggering actions based on object uploads.