Which of the following quantities does not change when two bodies moving at the same constant velocity collide?

Momentum is defined as the quantity of motion of the body. It is measured by “mass × velocity”, as momentum depends upon velocity, and it depends on the direction of the motion of the body as well.

When two bodies contact without any external force, the momentum remains constant because there is no force present, hence momentum does not change. However, the bodies' speed and velocity will be altered as a result of the impact.