Final answer:
The A/C ID on the DD 1801 form for an Army aircraft with serial number 57-9555 would likely include a shortened version of the serial number or an operational callsign, which is entered in item 7 of the form.
Step-by-step explanation:
The question pertains to the proper method for filling out a flight plan on form DD 1801 for a military aircraft, specifically where to input the aircraft's identification. In this instance, the serial number given is 57-9555. Based on military documentation practices, the A/C ID refers to the aircraft identification section of the form. For the DD Form 1801 (which is the DoD International Flight Plan), the relevant identifier would be entered in item 7 "A/C ID". This identifier is typically a shortened version of the serial number or an operational callsign assigned to the aircraft. Without specific guidance on abbreviations, the serial sequence itself or part of it might be used, ensuring it fits within any prescribed character limits on the form.