Explanation:

In Microsoft Defender for Cloud (formerly Azure Security Center), workflow automation allows you to automatically respond to security alerts and recommendations by triggering remediation actions.
When you create an Azure Policy to enforce automatic remediation based on Defender alerts or recommendations, the effect determines what the policy does when a resource is found noncompliant:
* DeployIfNotExists is the correct effect to use for automatic remediation. This effect automatically deploys a remediation task (such as a Logic App or other automation) when a matching noncompliant resource is detected. It's commonly used in Defender for Cloud to deploy missing security configurations or initiate an automated remediation workflow.
* Append only adds metadata or parameters to resources-it does not enforce or deploy remediation actions.
* EnforceRegoPolicy is used for container compliance with Gatekeeper policies (Kubernetes), not for Defender workflows.
For the automation mechanism:
* An Azure Logic Apps app with the trigger "When an Azure Security Center alert is created or triggered" is the correct choice. This Logic App acts as the workflow automation engine that runs whenever a new alert is raised. It can perform actions such as isolating VMs, disabling users, or notifying SOC teams.
* Using a trigger for "When a response to an Azure Security Center alert is triggered" would only activate after a manual response, not automatically.
* Automation runbooks with webhooks can be used for custom automation, but Defender workflow automation integrates natively with Logic Apps and not directly with runbooks.