Final answer:
Turning off the engine could possibly reset the ABS warning on a bus, but the warning may reappear if the issue persists. To properly address the ABS warning, a mechanic should diagnose and repair any issues.
Step-by-step explanation:
The anti-lock braking system (ABS) warning light on a bus or any vehicle is designed to alert the driver to a potential problem with the ABS. Among the options provided, turning off the engine (A. Turning off the engine) could potentially reset the system and cancel the ABS warning, but this is not guaranteed as the warning may reappear if the problem persists. Typically, options such as applying the brakes firmly, the speed of the bus exceeding 10 mph, and turning on the headlights will not cancel the ABS warning as they do not directly impact the ABS diagnostic system. The best action to take when an ABS warning light persists is to have the system checked by a qualified mechanic to diagnose and repair any issues.