Which of the following is NOT an action type for IoT Hub alerts?

In the context of IoT Hub alerts, an action type that is NOT associated with alerts would be Event Subscription. The reason for this is that IoT Hub alerts focus on immediate notification actions based on conditions that arise within an IoT solution, such as anomalies or threshold breaches.

The other options represent typical notification mechanisms used in response to alerts. Email, SMS/Push/Voice are common communication methods to notify users or system administrators about significant events. Secure Webhook allows for sending alerts to predefined endpoints in a secure manner, enabling integration with other applications or services. An Automation Runbook facilitates automated responses to alerts, allowing for predefined workflows to be executed in reaction to certain triggers.

In contrast, Event Subscription is generally a broader Azure resource feature that allows other services to subscribe to events emitted from a resource, rather than functioning as a direct action taken in response to IoT Hub alerts. Thus, it does not fit the criteria of an action type specifically for IoT Hub alerts.