What does a low air pressure warning buzzer and red dashboard lamp indicate when it activates in an air-brake equipped vehicle?

1. The vehicle has reached its maximum legal speed
2. System air pressure has fallen below safe operating levels (typically below 4.5 to 5.5 bar / 60 psi), requiring the driver to stop immediately before spring brakes lock the wheels
3. The cabin air conditioning filter is clogged
4. The fuel tank cap is slightly loose