What is the purpose of the primary air pressure gauge on the M915?

Answer: B — To indicate rear truck brakes

The primary air pressure gauge on the M915 serves to indicate the air pressure in the vehicle's braking system, which is crucial for the operation of the rear truck brakes. Proper air pressure ensures that the brakes function effectively, providing the necessary braking force for safe operation, especially given the size and weight of the vehicle. By monitoring the air pressure, operators can ascertain whether the braking system is functioning within safe parameters and detect any potential issues with air leaks or system malfunction. Options like monitoring tire pressure, displaying engine oil pressure, or checking fuel levels do not pertain to the purpose of the primary air pressure gauge. Each of those functions is monitored by different instruments within the vehicle, specifically designed to provide accurate readings for tires, oil, and fuel reserves, thereby highlighting the importance of specialized gauges for specific functions in vehicle operation.