What safety feature regulates the brake system's air pressure?

Answer: A — Pressure relief valve

The pressure relief valve is a crucial safety feature in the brake system, as it helps maintain the proper air pressure within the system. This valve functions by releasing excess air pressure if it rises beyond a safe level, ensuring that the air brake system operates within designed limits. Maintaining appropriate pressure is essential for the effective performance of the brakes, preventing mechanical failures, and enhancing overall safety while driving a large vehicle. The other options do not serve this specific regulatory function. The brake lever is the control used by the driver to engage the brakes but does not regulate air pressure. The service brake is the primary brake used for normal stopping, while the pneumatic pump's role is to supply the air pressure needed for braking rather than regulating it.