Explanation: Approved auto-alarm systems are designed with a critical fail-safe mechanism. This means that if a vital component, like a vacuum tube heater, fails, the system must automatically alert the operator. When the heater burns out, the vacuum tube ceases to function. This loss of operation is detected by a monitoring circuit. This circuit typically maintains a supervisory relay in an energized state as long as the system is healthy. Upon the heater's failure, the supervisory relay de-energizes. When this relay de-energizes, its normally closed contacts will **close**. These closed contacts then complete an independent circuit, which in turn **energizes the bell relay**, causing the alarm to sound. This design ensures that a system malfunction is immediately indicated. Therefore, "A pair of contacts close and energize the bell relay" accurately describes the fail-safe action to trigger the alarm. Options B, C, and D describe actions that would not reliably trigger a positive alarm indication in a fail-safe system.