Cloud kicks has a multi-screen flow that its call center agents use when handling inbound service desk calls. At one of the steps in the flow, the agents should be presented with a list of order…

While an Apex controller is used in Visualforce or Lightning components, it doesn't directly facilitate real-time external data retrieval for a Flow screen without the specific implementation of a REST callout.

An Apex REST class can be designed to make an HTTP callout to an external Order Management System (OMS) to fetch real-time data. This Apex logic can then be exposed as an invocable action for consumption by a Flow, allowing the Flow to retrieve and display dynamic data on a screen.

An outbound message is used for sending data *out* of Salesforce based on a record change, not for retrieving real-time data *into* a Flow for display.

An invocable method is the *interface* that allows a Flow to call Apex code, but it doesn't describe the *mechanism* for retrieving external data; the Apex class containing the HTTP REST callout logic (option B) is the core solution.