Final answer:
A low air pressure warning device should be visible and audible.
Step-by-step explanation:
The correct answer is d) visible and audible. A low air pressure warning device, such as in an airplane, is designed to provide both audible and visual warnings to alert the pilot about the low air pressure.
This is important for the safety of the passengers and the aircraft. The audible alarm can be a loud sound or a voice alarm to attract the pilot's attention, while the visual alarm can be a flashing light or a message displayed on a screen.