Correct Answer: DThe correct language used to define data structures within the web service's interface definition in a SOAP-based Mule application is XSD (XML Schema Definition). XSD is used to define the structure and data types for XML documents, and it is typically employed to specify the exact format of the messages exchanged between the client and the web service. While WSDL (Web Services Description Language) describes the overall web service interface, including operations and endpoints, the data structures themselves are defined using XSD within the WSDL document.