Which AWS service provides this functionality?

Explanations:

AWS IAM Identity Center (AWS Single Sign-On) is designed for SSO within AWS accounts and applications but does not directly provide social media or external account login capabilities.

AWS Config is a service for monitoring and auditing AWS resource configurations; it does not provide identity access management features.

Amazon Cognito allows users to sign in using social media accounts, email, or other identity providers, making it the right choice for identity access management in this context.

AWS Identity and Access Management (IAM) is focused on managing permissions and access to AWS resources, not for enabling user sign-ins via social media or other external accounts.