When should parking brakes be used in air brake vehicles?

Answer: C — Whenever the vehicle is parked

Parking brakes should be used whenever the vehicle is parked to ensure that the vehicle remains stationary and does not roll away. This is particularly important for air brake vehicles, as they rely on a different mechanism for regular braking. Using parking brakes helps to secure the vehicle, especially on inclines or uneven surfaces where gravity could cause it to move. It's a critical safety measure that prevents accidents and is a legal requirement in many jurisdictions. The other options suggest scenarios where parking brakes may not be appropriate or necessary. Like the assertion that parking brakes should be used during all driving conditions is impractical; they are specifically designed for parking. Similarly, suggesting use only during night driving overlooks the necessity of securing a parked vehicle at any time of day. Lastly, the idea of utilizing parking brakes when the vehicle is idling is not ideal as idling does not constitute parking, leading to potential confusion about the proper functioning of the vehicle's brake systems.