The correct answer is: Deploy a CloudFront distribution with the existing S3 bucket as the origin. Route download requests through CloudFront using signed URLs for access control.

Amazon CloudFront is a global content delivery network (CDN) that reduces latency and minimizes the cost of data transfer out of AWS by caching content at edge locations closer to end users. By configuring CloudFront to use the S3 bucket as its origin, data can be cached in locations near customers in North America and Europe, leading to faster downloads and reduced inter-Region data transfer costs. CloudFront also supports signed URLs, maintaining secure access to protected content without requiring major application changes.

The option Enable S3 Transfer Acceleration on the bucket and direct customer download requests to the acceleration endpoint using S3 signed URLs helps with speed, but not cost. S3 Transfer Acceleration routes traffic through Amazons edge network but comes with an extra charge for each accelerated transfer. It also lacks the global caching capabilities of CloudFront, making it less cost-effective at scale.

The option Create a second S3 bucket in eu-central-1 and configure S3 Cross-Region Replication. Serve requests from the bucket closest to the customer using S3 signed URLs is more complex and expensive. It incurs storage and replication costs, and managing which users access which Region adds operational overhead. Its also unnecessary when CloudFront can deliver content globally from a single origin.

The option Update the application to stream data directly to customers from S3 and handle access control in the application layer instead of using signed URLs introduces unnecessary architectural changes and complexity. Streaming large datasets may not be suitable for many customers, and offloading access control to the app increases development and security risks.

This question is important because it tests knowledge of performance optimization and cost efficiency in content distribution. When serving large static content (like datasets) to a global user base, CloudFront is almost always the right choice for minimizing latency and data transfer costs.

A helpful trick: look for keywords like 'reduce transfer cost,' 'improve performance,' and 'geographically distributed users.' These often point to Amazon CloudFront as the answer. Be cautious with tempting but overly complex or expensive options like Transfer Acceleration or multi-Region replication unless specifically required.

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