Which policy should be set to 'Enabled' to suppress all notifications in Microsoft Defender via Group Policy?

The correct choice is to set the policy to 'Enabled' for suppressing all notifications in Microsoft Defender to "Suppress all notifications." This option effectively disables any alerts, pop-ups, or messages that Microsoft Defender would typically display to the user, thereby creating a seamless experience without interruptions related to security notifications.

This setting is particularly useful in environments where you want to minimize distractions for users or control the notification flow centrally through Group Policy. By enabling this option, administrators can ensure that users remain focused on their work without being disturbed by security alerts.

The other options, while they address specific types of notifications, do not provide the comprehensive suppression of all notifications as required by the question. "Suppress Windows Defender notifications" would only address notifications from Windows Defender specifically, leaving other notifications untouched. "Suppress all alerts" similarly targets alerts but not necessarily all forms of notifications that could appear. "Suppress scan completion notifications" only addresses notifications related to scan completions, missing the broader aim of controlling all notifications.

By selecting "Suppress all notifications," you achieve the desired outcome of curtailing all communication from Microsoft Defender, which is essential for particular deployment scenarios where user experience is prioritized.