Which AWS service or feature should the developer implement to meet these requirements?

Explanations:

Amazon Cognito user pools is designed for user authentication and management, making it ideal for handling sign-ups, sign-ins, and storing user attributes. It scales to millions of users and provides easy integration with other AWS services.

AWS Mobile Hub User File Storage is intended for storing files like images, videos, etc., not for user account management or handling user authentication.

AWS AppSync is a fully managed service for real-time data synchronization, focusing on GraphQL APIs. It is not suitable for user authentication and attribute storage.

AWS Mobile Hub Cloud Logic focuses on providing backend services for mobile apps, including APIs and business logic. It does not provide user authentication or user attribute storage out-of-the-box.