Under what condition will the audible warning horn be inoperative?

The correct choice indicates that the audible warning horn will be inoperative if either the #1 or #2 touchdown relay circuit breaker is open. This condition leads to a situation where the signals necessary for the operation of the warning system are disrupted, thus rendering the audible warning horn inoperative. The touchdown relays are crucial components in the aircraft systems, and if there's a fault or if they are opened, it compromises the functions they support, including warnings that alert the crew to certain operational conditions.

Understanding the importance of these relays helps clarify why any break in the circuit directly impacts the warning system. The touch down relays typically serve as a safety mechanism that provides real-time feedback regarding the aircraft's status, and when they're not functioning due to an open circuit breaker, the system has no way to generate audible alerts.

Further context suggests that in scenarios where other conditions like having the airplane on jacks or the APU power switch being off are considered, those do not inherently create a direct link to disabling the warning horn. The integrity of the touchdown relay system is fundamental to the warning function, which is why the focus on their circuit breakers is critical in this question.