Which AWS service provides user sign-up and authentication capabilities for web and mobile applications?

Amazon Cognito is the correct answer because it is a fully managed service that provides user sign-up, sign-in, and access control for web and mobile applications. It supports millions of users and integrates with various identity providers. AWS Config is incorrect because it provides a detailed inventory of your AWS resources, their configurations, and how they change over time. It's for auditing and compliance, not user authentication. Amazon GuardDuty is incorrect because it is a threat detection service that continuously monitors your AWS accounts and workloads for malicious activity and unauthorized behavior. It focuses on security monitoring. AWS Systems Manager is incorrect because it helps you gain operational insights and take action on your AWS resources. It's used for managing and automating operational tasks across your AWS infrastructure.