Which AWS service should the company use to meet these requirements?

Explanations:

AWS Single Sign-On is primarily designed for managing access to multiple AWS accounts and applications through a single interface, not specifically for user authentication directly via username/password or third-party providers.

AWS Signer is a service for signing code, ensuring integrity and authenticity, not for user authentication. It does not support user sign-in mechanisms.

Amazon Cognito provides user authentication and can handle sign-in with a username/password as well as support for third-party identity providers like Google, Facebook, etc., making it suitable for the described requirements.

AWS Directory Service is mainly used for creating and managing Microsoft Active Directory in the cloud and is not focused on direct user authentication via usernames and passwords or integration with third-party providers.