Final answer:
The low pressure warning devices on large buses commonly signal at 60-65 PSI, which is crucial for the safe operation of the vehicle's air brake system.
Step-by-step explanation:
On large buses, it is common for the low pressure warning devices to signal at 60-65 PSI. The correct answer to this question is a. 60-65 PSI. It is very important for bus drivers to pay attention to these warnings as they are crucial for the safe operation of the vehicle's air brake system. Operating a vehicle with insufficient air pressure in the braking system can lead to brake failure and pose serious safety risks.