The correct answer is C: AWS Systems Manager Parameter Store. This is because Parameter Store is a cost-effective solution for securely storing and managing configuration data such as license keys. It provides a SecureString option which encrypts the data, ensuring secure access. Unlike AWS Secrets Manager, which incurs additional costs ($0.40 per secret per month plus data retrieval costs), Parameter Store comes with no additional cost for standard parameters, making it the most cost-effective solution for this use case.