What provides the force to push the brake cylinder piston back when brakes are released?

The correct answer is that a spring provides the force to push the brake cylinder piston back when brakes are released. In train braking systems, when the brakes are applied, air pressure is used to engage the brake cylinders, pushing the brake shoes against the wheels to slow down or stop the train. Once the driver releases the brakes, it is the mechanical action of the spring that returns the brake piston to its original position. This spring force ensures that the brake shoes are disengaged from the wheels efficiently and promptly, allowing for safe and reliable train operation.

Other options like air pressure and hydraulic fluid are primarily involved in applying the brakes rather than retracting them. Gravity may play a role in other aspects of the train's operation, but it does not directly influence the brake piston retraction mechanism.