A company runs an application on Amazon EC2 instances in a private subnet. The application needs to store and retrieve data in Amazon S3 buckets. Regulations require that the data must not traverse the public internet. What should a solutions architect implement MOST cost-effectively?

An S3 gateway endpoint allows instances in a private subnet to access S3 buckets without traversing the public internet, fulfilling the regulatory requirement. It is also the most cost-effective solution among the options because there are no additional charges for using gateway endpoints. A NAT gateway would allow internet access but would incur data processing charges and does not guarantee that traffic stays within the AWS network for S3. AWS Storage Gateway is a hybrid cloud storage service, not primarily designed for direct private access from EC2 to S3 within AWS, and would be more complex and expensive. An S3 interface endpoint (powered by AWS PrivateLink) also provides private access, but it incurs hourly charges and data processing charges, making the gateway endpoint more cost-effective for S3.