Which AWS service will meet these requirements MOST cost-effectively?

Explanations:

Amazon EBS is designed for block storage and is primarily used with EC2 instances. While it supports data encryption at rest and offers high availability, it does not provide direct access over the internet without EC2, making it less suitable for this scenario.

Amazon S3 is an object storage service that is highly available and durable, supports encryption at rest, and allows direct access over the internet via a simple API. This makes it the most cost-effective choice for the given requirements.

Amazon EFS is a scalable file storage service that can be accessed from multiple EC2 instances. It supports encryption at rest and is highly available, but it does not provide direct internet access to data, limiting its applicability for this requirement.

AWS Storage Gateway provides a way to connect on-premises environments to cloud storage but is more focused on hybrid storage solutions. It does not provide direct internet access to data and may introduce additional costs and complexity compared to S3.