When should the low-pressure warning device alert the driver?

Answer: B — If pressure falls below 60 psi

The low-pressure warning device is designed to alert the driver when the air pressure in the brake system falls to a critical level that could affect braking performance. In this case, it is set to activate when the pressure drops below 60 psi. This threshold is established to ensure that the driver is warned at a point where the system is still capable of safely operating, but low enough to prompt immediate attention and corrective action, such as checking for leaks or other issues. Being aware of this warning allows drivers to address any potential problems before they compromise the safety of the vehicle and its occupants. Lower thresholds, like those indicated in other choices, would not provide an adequate safety margin, as the braking system could already be at risk of failing before the driver is alerted.