Explanation: To enable Salesforce to call external services within a flow, the appropriate specifications must be imported to make these external services available as invokable actions. The correct specification isOpen API specifications. OpenAPI Specification (formerly known as Swagger) is a standard for defining APIs which can be easily imported into Salesforce to facilitate the integration and invocation of external services.
Import OpenAPI Specifications: Salesforce allows the import of OpenAPI specifications, which define the available endpoints and operations of an API, making them accessible as invocable actions in a Salesforce flow.
Creating External Services: By importing an OpenAPI definition into Salesforce, it creates an External Service, which you can then use within Flow to interact with the defined APIs.
Flow Integration: Once imported, these services can be integrated into Salesforce Flows to automate processes that require interaction with external systems. :Salesforce External Services and API Integrations