Which car would appear to be moving the slowest to a passenger on the train?

For B and D, if botch cars move with same velocity, both the drivers will see passenger moving with the same velocity.

Now suppose the train is moving at velocity v, therefore, with respect to gound, velocity of passenger will also be v since your frame of refrence(Earth) is not under acceleration. But for drivers, since car is under acceleration, velocity of passenger will be more than v. if motion of the car is opposite to that of the train. Here I should also say that this velocity is relative velocity because in physic, study of the motion of particle is done with respect to some frame of refrence.

So in your question, the passenger will appear slowest for C.