The back-up timer terminates the exposure when the total milliampere-seconds (mAs) has reached:

The back-up timer is a safety feature used in radiography that is designed to limit the duration of an exposure in instances where the automatic exposure control (AEC) system may fail to terminate the exposure correctly. The back-up timer stops the exposure when a predetermined amount of milliampere-seconds (mAs) has been reached.

The typical threshold set for back-up timers is generally around 600 mAs, which serves to protect the patient from excessive exposure especially in situations where the AEC may not function as intended. This threshold is chosen to ensure that even in the event of a malfunction, the exposure remains within a safe range while still being sufficient to achieve an adequate diagnostic image.

While other options suggest different mAs values, they do not reflect the commonly established safety limits associated with back-up timers in radiographic practice. In this context, 600 mAs is the standard figure that balances safety and adequate imaging needs, which is why it is the correct answer for this question.