Which type of API is defined by a WSDL file?

The correct choice is SOAP APIs, as they are specifically designed to operate according to the definitions provided in a Web Services Description Language (WSDL) file. WSDL is an XML-based language that describes the functionalities offered by a web service, detailing how to interact with it, including information on the service's available methods, input and output formats, and the protocol used for communication.

SOAP APIs rely on a standard messaging protocol that defines how requests and responses are structured, making them inherently different from REST APIs, which do not use WSDL for their definitions. Instead, REST APIs typically rely on other means, such as OpenAPI specifications, to define their endpoints and data structures.

API spec fragments and connectors do not inherently conform to a WSDL definition, as they refer to different concepts in API management and integration. API spec fragments represent parts of an API specification, while connectors are used to facilitate integration with third-party services, usually not dependent on WSDL. Thus, SOAP APIs are uniquely defined by WSDL files, making this the correct answer.