AWS Certified Cloud Practitioner

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.