A. Publish the error event using the Eventbus.publish() method and have the external system subscribe to the event using CometD.

Here’s why this option is correct:

1. Eventbus.publish(): This method is used to publish platform events, which are ideal for broadcasting error events to external systems. Platform events are designed for event-driven architecture and can be easily handled within Salesforce and subscribed to by external systems.
2. CometD: This is a protocol that allows external systems to subscribe to real-time events from Salesforce. CometD is commonly used for streaming API and platform events, making it suitable for this requirement.

Using Eventbus.publish() ensures that the error event is published correctly within Salesforce, and subscribing via CometD allows the external system to receive these events in real-time, thus fulfilling the requirement of notifying the external system when an unhandled exception occurs.