In GraphQL, the blank is filled by "Query Language." GraphQL allows clients to define precisely the data they require, eliminating over-fetching or under-fetching, and this is achieved through a query language designed specifically for this purpose.

When deciding between REST, SOAP, or GraphQL for a new API, crucial considerations include data serialization format, platform compatibility, and the need for strict versioning. These factors can significantly impact how data is transmitted, processed, and maintained in the API.

Developers might need to create custom HTTP methods for their Web APIs in scenarios where the standard HTTP methods (GET, POST, PUT, DELETE, etc.) lack the expressiveness or functionality required for their specific use case. Creating custom methods is allowed within the HTTP standard, but it should be done judiciously and documented well to ensure clarity. They are typically used when there is a genuine need for additional, non-standard functionality that cannot be achieved using the standard methods.

When changes to an API are not backward compatible, versioning can help in preventing disruptions to existing clients. API versioning allows the introduction of new features and changes without affecting existing clients, making it easier to manage changes while maintaining compatibility with older versions.

When designing an API for high-traffic applications using ASP.NET Core, several considerations are crucial. Options A, B, and D are key considerations. Caching mechanisms can help reduce the load on your server by serving cached responses, input validation and sanitization are essential for security, and load balancing and horizontal scaling are necessary to handle high traffic efficiently. Synchronous I/O operations should generally be avoided to maintain responsiveness.

B. Reviewing error logs and tracing requests to identify patterns and potential issues is an essential troubleshooting step for resolving intermittent failures in an API. This allows you to pinpoint the issues and take corrective actions. Rolling back to a previous version or ignoring the problem are not proactive solutions. Waiting for users to report problems can lead to poor user experiences.

Integration testing in APIs differs from unit testing because it focuses on testing the interactions between different components or modules, rather than testing a single isolated component. Integration tests verify that various parts of an application work together correctly, helping to detect issues related to data flow and communication between components. Unit testing, on the other hand, focuses on testing individual units or functions.

Alternatives to API keys for authentication include OAuth, which may offer more secure and granular access control. OAuth is a robust and standardized protocol that allows applications to access resources on behalf of users, providing better security and control than traditional API keys.

Data transfer efficiency differs between REST and gRPC due to their underlying communication protocols. gRPC uses a binary protocol, Protocol Buffers, which is more efficient in terms of data size and speed compared to REST, which often uses text-based formats like JSON or XML.

A successful SQL Injection attack on an API can result in unauthorized access to data and potential data corruption. Attackers can manipulate SQL queries to gain access to sensitive information or even modify the data within the database, posing a significant security risk.