What is the most common cause of parking brake failure?

Answer: B — Driving with the parking brake on

The most common cause of parking brake failure is driving with the parking brake on. This can cause the brake pads to wear down prematurely and can also cause damage to the brake system. While the other options may also lead to parking brake failure, they are not as common as driving with the parking brake engaged. Lack of maintenance may lead to issues with the brake system, but is not necessarily the main cause of parking brake failure. Rusty cables and fluid leakage are also potential causes of parking brake failure, but they are less likely to occur compared to driving with the parking brake on.