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

Momentum is equal to the force required to bring an item to a stop in a unit amount of time and is equal to its mass multiplied by its velocity. The total momentum of any array of many items is the sum of the individual momenta.

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.