Answer is correct, D.

An action group is a collection of actions that are triggered by an Azure alert. In this scenario, you need to create an alert rule that will run App1 if VM1 stops, and for this purpose, you need to create an action group. An action group defines the set of actions to be taken when an alert is triggered, such as running an Azure function, sending an email, or creating an Azure ticket.

By creating an action group and associating it with the alert rule, you can automate the process of running App1 if VM1 stops, without the need for manual intervention. This helps ensure that critical systems, such as App1, are automatically activated when necessary, improving the overall reliability and availability of your Azure services.

D. an action group