Which AWS service meets these requirements?

Explanations:

Amazon RDS for MariaDB is a relational database service, which is not ideal for unstructured data and may struggle with scaling to millions of concurrent users due to its limitations on horizontal scaling and session management.

Amazon DynamoDB is a fully managed NoSQL database service that can handle unstructured data and is designed for high scalability and low latency, making it suitable for applications with millions of concurrent users.

Amazon RDS for SQL Server is also a relational database that is less optimal for unstructured data and may face performance bottlenecks under high concurrency due to its need for vertical scaling.

Amazon Redshift is a data warehousing service optimized for analytics, not for real-time transactions or unstructured data handling, and is not suitable for applications requiring low latency and high concurrency.