Explanation: An auto-alarm system is designed to alert the operator to critical events, which includes both the receipt of the emergency signal and any condition that prevents the system from functioning correctly. * **A) Receipt of the true auto-alarm signal** is the primary function: the square-law detector processes the signal, and the mechanical selector verifies the specific two-tone sequence. **Low battery voltage** would trigger an alarm as it indicates a power fault, compromising the system's reliability. * **B) A filament circuit open** in older tube-based receivers means the receiver is inoperable, prompting a fault alarm. **An open 24 V circuit** also indicates a power supply failure, rendering the system inactive and thus triggering an alarm. * **C) A blown battery charger fuse** leads to eventual battery depletion (as in A). A **motor stopped or running incorrectly** within the mechanical selector indicates a critical mechanical failure, preventing proper operation. Therefore, an auto-alarm system alerts operators to both the desired signal and various internal failures that prevent it from performing its crucial safety function.