When should the parking brake not be set?

The correct answer is that the parking brake should not be set if hot brakes are suspected. When brakes are hot, applying the parking brake can cause brake components to warp or fail due to overheating. This scenario may happen after a prolonged landing rollout or taxiing under heavy braking. Setting the parking brake in this condition can restrict airflow and cooling to the brakes, exacerbating the problem.

While the other options present situations when the parking brake may not be advisable, they are not as critical in the context of immediate safety and aircraft performance. For example, during flight, the parking brake is typically not applicable since the aircraft is in motion in a way that doesn't require a parking brake, but there's no immediate risk like there is with hot brakes. Sudden stops might require careful consideration of brake usage, but they don't carry the same risk level as hot brakes. Lastly, low temperatures and moisture can lead to ice formation, impacting brake performance, but this consideration doesn't equate to the severe risk presented by setting a parking brake on hot brakes.