Rate limiting in a Web API is implemented to protect against abuse and ensure fair usage. It limits the number of requests that a client can make within a specified time frame, preventing overuse and ensuring that resources are available for all users. This helps maintain the quality of service and prevents misuse.

The REST (Representational State Transfer) architectural style for Web APIs uses a stateless communication mechanism, ensuring that each call from a client to a server is treated as a new request. RESTful APIs are known for their simplicity and scalability.

Automated testing for APIs can present challenges in terms of limited scalability, as maintaining test automation can become complex as the number of APIs grows. It is essential to ensure that the test automation framework can handle the increasing number of API endpoints and scenarios.

The purpose of debugging in API development is to identify and fix issues or errors in the API code and functionality. Debugging helps ensure that the API works as intended, and it is a critical step in the development process to deliver a reliable and error-free API to users. Debugging does not aim to introduce errors, enhance security, or slow down the API.

API security concerns include protecting against unauthorized access, SQL injection, and data breaches. SQL injection is a type of attack where an attacker injects malicious SQL code into input fields, potentially compromising the database and exposing sensitive information.

Monitoring a critical API requires tracking relevant metrics. Prioritizing error rates and system resource utilization (Option 3) is the most appropriate choice, as it helps identify issues and performance problems. Response times are essential but should not be the sole focus. Option 1 suggests ignoring anomalies, which is not advisable, and option 2 is less relevant to anomaly response. Option 4 lacks the essential focus on error rates and resource utilization.

The correct approach to securely store and use an API key is to utilize environment variables or a secure key management system. Storing API keys in plain text or in version-controlled files is insecure and should be avoided. Encrypting and storing in a restricted database could be an option but may not be as secure as using dedicated key management solutions.

The primary purpose of API authentication is to ensure data privacy and security. It verifies the identity of the user or application requesting access to an API, preventing unauthorized access and protecting sensitive data. This security measure is vital in modern web development to safeguard user information and maintain the integrity of the API.

To keep the user interface updated in a social media application using GraphQL, you should use GraphQL subscriptions. Subscriptions enable the server to push real-time updates to clients over WebSocket connections, providing efficient and responsive real-time communication.

OAuth 2.0 is primarily used for authentication and authorization in web applications. It allows a user to grant limited access to their resources on one site to another site without sharing their credentials. It is commonly used for user authentication and secure data access between different web applications.