Which aspect enhances the visibility of an invocable action in Salesforce tools?

The visibility of an invocable action in Salesforce tools is significantly enhanced by adding an InvocableMethod annotation. This annotation specifically indicates that the method can be called from other processes, such as Process Builder or Flow, thereby making it discoverable and usable within these automation tools. By marking a method with this annotation, you allow it to participate in the broader platform functionality where it can be invoked by other components, facilitating the integration and automation of business processes.

The other choices do not directly enhance visibility in the same way. For instance, defining input variable types helps with the method's functionality but does not explicitly make the method visible to other tools. Creating a custom user interface pertains more to user experience and does not relate to the visibility of invocable actions. Connecting to external data sources expands data capabilities but does not directly influence how invocable actions are recognized within the Salesforce environment.