What must happen for a vehicle to pass the air pressure warning device test?

For a vehicle to pass the air pressure warning device test, it is essential that all specified criteria are met, which is why the most comprehensive option is the correct one.

The air pressure gauge functioning properly is crucial because it gives the driver accurate information about the air pressure levels within the braking system; this helps ensure the driver is aware of whether the pressure is adequate for safe braking performance.

Furthermore, the mechanism activating before pressure reaches specified levels serves as a critical safety feature. It alerts the driver to a decline in air pressure that could indicate potential issues with the braking system, allowing for timely corrective action before safety is compromised.

Additionally, having both visual and audible warning devices ensures that the driver receives redundant alerts, catering to both sight and hearing, which is especially important in various driving conditions where one sensory alert may be missed.

Thus, all these components are integral to the function and safety of the air pressure warning system, leading to the conclusion that all of the listed elements must function correctly for the test to be passed.