In this case, the best course of action would be to B. Shut off your engine.
Answer options A, C, and D all allow for the car to remain in motion. By putting the vehicle in reverse, the stuck accelerator pedal would cause the car to rapidly move backwards. Both shifting to neutral and switching to a lower gear would result in the car continuing to move forward as a result of the malfunctioning accelerator. The safest option that would maintain the inter workings of the car and stop any uncontrolled motion would be to shut off the engine and address the problem.
I hope this helps! :)
