Amazon Cognitois an AWS service that provides user sign-up, sign-in, and access control to web and mobile applications. It supports authentication for different identity providers, including social identity providers (such as Google, Facebook, and Apple), enterprise identity providers via SAML 2.0, and its own user pools.

Amazon Cognito offers:

User Poolsfor managing user registration, authentication, and account recovery.

Federated Identitiesfor managing user access from external identity providers.

Why other options are not suitable:

B. AWS Config: A service for tracking resource configuration changes, not related to user authentication.

C. Amazon GuardDuty: A threat detection service, not related to user sign-up or authentication.

D. AWS Systems Manager: A service to manage AWS resources, but it does not provide user authentication.