Answer: Option B: The vertical velocity of projectile Y is changing, and the horizontal velocity of projectile X is constant.

In a projectile motion, there is no force acting in the horizontal direction. Hence, in the absence of any external force, the body would continue to move at constant velocity and hence, there would be no acceleration in the horizontal direction.

In the vertical direction, due to gravitational pull, the body would experience acceleration due to gravity(g=-9.8 m/s^2) which would change the vertical velocity.

Hence, the correct option would be B only- The vertical velocity of projectile Y is changing, and the horizontal velocity of projectile X is constant.

In option A, it is wrongly said that horizontal velocity is changing.

In option C, again it is written that there would be horizontal acceleration of projectile X.

In option D, it is written that there would be horizontal acceleration of projectile Y which is wrong.