A body is moving in a room with a velocity of 20 m/s perpendicular to the two walls separated by 5 meters. There is no friction and the collisions with the walls are elastic. The motion of the body is

Body collides elastically with walls of room. So, there will be no loss in its energy and it will remain colliding with walls of room, so its motion will be periodic. There is no change in energy of the body, hence there is no acceleration, so its motion is not SHM.