To retrieve data from an external system in real time during a flow, the developer should use an invocable method.
Option B: An invocable method
public with sharing class OrderService {
@InvocableMethod(callout=true)
public static List<OrderInfo> getOrderData(List<String> inputs) {
// Callout to external system and return data
}
}
Reference:
"To invoke Apex methods that perform callouts from a flow, use the @InvocableMethod annotation with callout=true."
- Salesforce Developer Guide: Invocable Methods
Why Other Options Are Incorrect:
Option A: An Apex controller is typically used with Visualforce pages or Aura components, not directly with Flows.
Option C: An outbound message sends data to external systems but does not retrieve data.
Option D: An Apex REST class exposes endpoints for external systems to call into Salesforce, not for Salesforce to call out.