A uniform metre stick is placed vertically on a horizontal frictionless surface and released. As the stick is in motion, the centre of mass moves

• At the origin, a metre stick is positioned vertically on a non-stick surface. 
• The centre of mass falls along the direction of the external force that is moving vertically down because it is a smooth plane and only gravitational force is acting on it in that direction.

Hence the correct answer is vertically down.