Hi Karsang! 

Greetings! 

So if the train is moving with constant velocity and a stone is thrown by him stone will gain a horizontal initial velocity equal to the velocity of train and as there is no acceleration of stone in horizontal direction(neglecting air resistance so it will move on with constant horizontal velocity but it will also experience gravitational force downward. So for the passenger it will look like it is just going down in a Straight line as both are going with same horizontal velocity but for person on ground will feel that it is doing projectile motion in a Parabolic Path. 

So Straight Line.

Please upvote if this was helpful to you. 

Enjoy your Physics! 

Hello Karsang, 

Then answer to your question is straight line motion and that is option A. 

I hope you will find my answer helpful. 

All the best.