Your organization wants to develop a mobile application backed by a secure and scalable API. The application will make use of user authentication, a database, and machine learning functionalities. Which combination of GCP services is best suited for this requirement?

*Explanation: * _Firebase for mobile backend and Cloud Endpoints for API._ Firebase provides mobile backend services, including user authentication, and Cloud Endpoints can secure and manage the API. This combination is well-suited for developing a secure and scalable mobile application with API functionality. Other options either lack the necessary mobile-specific backend or use complex solutions that are not as well-tailored for the scenario.