Which storage solution meets these requirements MOST cost-effectively?

Explanations:

Amazon Elastic Block Store (EBS) is designed for block storage and is tightly coupled with EC2 instances. It is not ideal for retaining large volumes of data for compliance purposes due to its cost and the fact that it is not designed for long-term data storage. Additionally, EBS volumes do not allow for concurrent access from multiple instances, which is a requirement for the reporting tool.

Amazon Elastic File System (EFS) is a managed file storage service that allows concurrent access from multiple EC2 instances. However, while it supports scalability and shared access, it is generally more expensive than Amazon S3 for long-term storage, especially when retaining data for 7 years, making it less cost-effective for this use case.

Amazon EC2 instance store provides temporary storage directly on the host machine and is ephemeral in nature. Data on instance store volumes is lost when the instance is stopped or terminated. This option does not meet the requirement for long-term retention of log files.

Amazon S3 is an object storage service designed for durability and scalability, making it suitable for retaining large volumes of data for extended periods, such as the required 7 years. S3 also supports concurrent access, allowing the reporting tool to analyze log files without issues. It is the most cost-effective solution for long-term storage compared to EBS or EFS.