In an isolated system, the total momentum is conserved. Therefore, the motion of the center of mass of the system remains unaffected by the internal collisions of the objects within it.
The correct response is: The velocity of the center of mass of the system does not change even as collisions occur.
This is because, while the individual velocities of the objects may change due to collisions, the overall motion of the center of mass remains constant unless acted upon by an external force.