According to the GDPR, in what situation must data subjects always be notified of a...

When personal data is processed at a facility of the processor that is not located within the borders of the EEA. Incorrect. The location where the data is processed is of no significance to the obligation to notify data subjects of personal data breaches.

When personal data is processed by a party that agreed to the draft processing contract but has not yet sign it. Incorrect. Personal data processed by another party than the controller without a valid written contract is considered a personal data breach. In the given situation however, negative consequences for the data subjects are unlikely. Notifying the data subject is not obligatory in that case.

When the system on which the personal data is processed is attacked causing damage to its storage devices. Incorrect. Damage to storage devices will make access to the data difficult or even impossible but does not imply illegal processing.

When there is a significant probability that the breach will lead to a high risk for the privacy of the data subjects. Correct. If there is a significant probability of negative impact on the data subjects, the controller is obliged to notify them of the breach. (Literature: A, Chapter 5)