What becomes inoperative with a failure of one of the remaining flight control computers or IRUs after all generators have failed?

The correct answer is that yaw damping becomes inoperative with the failure of one of the remaining flight control computers or Inertial Reference Units (IRUs) after all generators have failed.

In the C-17, yaw damping is a function that stabilizes the aircraft's yaw motion, which is particularly significant during flight in maintaining directional control. This yaw damping relies on inputs from the flight control computers and IRUs to function effectively. If all generators are lost, power to these systems is cut off, leading to a loss of yaw damping capability. This situation can affect the pilot's ability to maintain control over the aircraft's yaw dynamics, particularly during critical phases of flight such as approach or landing.

Other systems mentioned, like the flaps, landing gear, and mission computer, are supported by different architectures or backup systems that may allow them to remain functional despite certain failures. Flaps and landing gear systems often have mechanical backups or alternate power sources that could potentially allow them to operate even if some control computers fail. The mission computer may continue to function without full redundancy, depending on configuration and operating parameters. Therefore, yaw damping is uniquely affected by this specific loss of power and redundancy.