B. An invocable method : We are using an external system , so we'll be making an HTTP request to an External API . and this is doable via Invocable method .

D is correct indeed
Explanation:
An Apex REST class is a custom class that defines methods that can be invoked via HTTP requests. The developer can use the @RestResource annotation to expose the class as a REST resource, and use the @HttpGet, @HttpPost, @HttpPut, or @HttpDelete annotations to define the methods that correspond to the HTTP verbs. The developer can also use the RestContext class to access the request and response information, such as the parameters, headers, and body. An Apex REST class can be used to integrate with an external order management system by sending and receiving JSON or XML data, and parsing and serializing it using the built-in Apex methods. The developer can then use the flow:callApex action in the flow to invoke the Apex REST class and pass the data to the screen element. An Apex REST class is different from the other options listed, as they are not suitable for this requirement. An Apex controller is a class that provides logic and data for a Visualforce page or a Lightning component, not a flow. An outbound message is a message that is sent from Salesforce to an external system when a workflow rule is triggered, not when a flow is executed. An invocable method is a method that is annotated with @InvocableMethod and can be called from a process, a flow, or a REST API, but it cannot make HTTP requests to an external system. References:

Apex REST Methods from the Apex Developer Guide
Call Apex from Flow from the Salesforce Help
Integrate with External Systems Using Apex REST Services from the Salesforce Trailhead...