The force necessary to apply the brakes on a subway car is supplied by which of the following?

The force necessary to apply the brakes on a subway car is primarily supplied by air compressors, which generate compressed air that is distributed to various components of the braking system. In pneumatic braking systems, this compressed air is used to activate brakes that work by pushing brake shoes against the wheels or rotating discs.

By using air pressure to create the force required for braking, the system can respond quickly and effectively when the operator engages the brakes. This is especially important in a subway system where rapid stops and starts are necessary to ensure safety and efficiency.

The other options, while related to mechanical systems, do not serve as the primary force source for braking in this context. Electric motors, for instance, are more commonly used for propulsion rather than for braking force. Hydraulic systems also play a role in certain types of brakes but are not the standard for most subway applications. Pneumatic cylinders indeed utilize compressed air, but they are components of the wider pneumatic braking system rather than the source of air compression itself.

Thus, air compressors are integral to supplying the necessary force for braking in subway cars, underpinning the operation of an effective and responsive braking system.