What type of file is provided to third-party developers for writing integrations with web services?

The correct answer is C, WSDL file. A WSDL (Web Services Description Language) file is specifically designed for describing the functionalities offered by a web service. It provides a machine-readable description of how the service can be called, what parameters it expects, and what data structures it returns.

When third-party developers want to integrate with a web service, the WSDL file serves as a contract that outlines the service's operations, input and output message formats, and the endpoints. This allows developers to generate code automatically or manually based on the definitions in the WSDL, making it easier to create applications that can interact seamlessly with the web service.

In contrast, JSON and XML files are primarily data formats used for exchanging information, not for describing web services themselves. While they can certainly be part of the data being sent to or received from a web service, they do not contain the necessary interface definitions and service behavior that a WSDL file provides. An HTML file, on the other hand, is meant for web page content and lacks the specifications for web services. Therefore, WSDL is the appropriate file type for third-party developers working on integrations with web services.