Which AWS service should the developer use to start building the application?

Explanations:

AWS Organizations is primarily used for managing multiple AWS accounts, not for building applications.

AWS Identity and Access Management (IAM) is used for managing permissions and access to AWS resources, but it does not directly assist in application development.

AWS Billing and Cost Management is focused on tracking and managing costs associated with AWS usage, not on application development.

Amazon Cognito is designed to handle user authentication and access for web and mobile applications, making it suitable for a developer starting to build a web application.