To which of these does the double foot valve directly apply pressure?

The correct answer pertains to the function of the double foot valve in a braking system. The double foot valve is crucial as it controls the application of air pressure to the brake system, ensuring that when the driver applies the brake, the appropriate pressure is delivered to specific axles.

In this context, the steering axle brakes are typically the ones affected directly by the double foot valve. This component allows for the modulation of air pressure that is required for braking, and with the steering axle being essential for vehicle control and maneuverability, it's critical that these brakes are responsive. By ensuring that adequate pressure is applied, the double foot valve plays a significant role in the braking efficiency and safety of the vehicle.

The other axle options and trailer brakes are typically managed by different systems within the overall air brake configuration. Therefore, the emphasis on the steering axle highlights the importance of precise braking control in maintaining both safety and vehicle performance on the road.