Which solution will meet these requirements MOST cost-effectively?

Explanations:

Amazon S3 can store encrypted files, but it does not provide native mechanisms for fine-grained access control or secure management of license keys specifically within automation scripts. Additionally, S3 would require custom implementation for access control and secret retrieval in automation scripts, increasing complexity and cost.

AWS Secrets Manager is a good choice for managing sensitive information, but it is more costly than other solutions. The requirement specifies a focus on cost-effectiveness, and while it does provide advanced features like automatic rotation, it may not be the most economical option for simple license key management.

AWS Systems Manager Parameter Store SecureString parameters allow for secure storage of sensitive information like license keys. It integrates well with EC2 and CloudFormation, providing a cost-effective solution with built-in encryption and access controls, meeting the automation script requirements efficiently.

CloudFormation NoEcho parameters can hide sensitive information in the CloudFormation outputs, but they do not provide a secure storage solution for license keys. This option lacks the capability to securely manage and access the keys outside of CloudFormation stacks, making it unsuitable for the requirement of automation scripts.