The correct answer is option 1: acceleration.

When a passenger in a moving train tosses a coin, the coin will have the same forward motion as the train. However, it will also experience the force of gravity pulling it downward. As a result, the coin will follow a curved trajectory, eventually falling behind the passenger.

If the train is moving with a uniform speed (option 3) or velocity (option 4), the coin should fall straight down, directly under the passenger. This is because the forward motion of the train is constant, and there is no acceleration acting on the coin.

However, if the coin falls behind the passenger, it indicates that there is a change in acceleration. This change in acceleration can occur when the train is accelerating (option 1) or decelerating (option 2). In both cases, the coin falls behind the passenger, demonstrating that the train is not moving with a uniform speed or velocity.

In summary, if a coin tossed in a moving train falls behind the passenger, it means that the train is accelerating or decelerating, making option 1 (acceleration) the correct answer.