Final answer:
The Ambari Alert Framework is the Ambari component that notifies the administrator if a Hadoop node goes down. It monitors the cluster and generates alerts for any detected issues, enabling administrators to respond accordingly.
Step-by-step explanation:
When a Hadoop node goes down, the Ambari Alert Framework is responsible for notifying the administrator of this event. The Ambari Alert Framework continuously monitors the health and status of the Hadoop cluster, including all the nodes and services running on it. If an issue is detected, such as a node failure, it issues alerts and notifications to provide administrators with the information necessary to take appropriate action.
The Ambari Alert Framework is integrated into the Ambari Web UI, where it provides a visual indication of any alarms or alerts generated due to a node's status change or failure among other system criteria. Therefore, the correct answer to the question is: D. Ambari Alert Framework.