Answer: Amazon Cognito

## Explanation Amazon Cognito is the correct choice for managing user sign-in for mobile applications with thousands of users because: - **Amazon Cognito** is specifically designed for mobile and web applications that need user authentication and authorization - It provides user pools that act as user directories with built-in sign-up and sign-in functionality - Supports social identity providers (Google, Facebook, etc.) and enterprise identity providers - Scales automatically to handle thousands of concurrent users - Provides secure token-based authentication **Why other options are incorrect:** - **AWS Directory Service**: Primarily for integrating with Microsoft Active Directory for enterprise directory services, not optimized for mobile app user authentication - **AWS IAM Identity Center**: Used for centralized access management to multiple AWS accounts and business applications, not designed for mobile app user authentication - **AWS IAM**: Used for managing access to AWS resources and services, not for end-user authentication in mobile applications For mobile applications requiring user authentication and management, Amazon Cognito is the recommended AWS service.

Author: Ritesh Yadav