Question

What does the Law of Conservation of Momentum state?

The net momentum of a system is the same before and after the collision.

There will always be a difference of the momentums before or after a collisions

No external (extra) force is needed for there to be a change in the momtentum of a system

For every action, there is an equal and opposite reaction

Answer 1

The correct statement is the first one: "The net momentum of a system is the same before and after the collision."

More precisely: the total (vector) momentum of an isolated system remains constant in time — i.e., the total momentum before an interaction equals the total momentum after — provided no external net force acts on the system.

Notes on the others:
- "For every action, there is an equal and opposite reaction" is Newton’s third law, which is related (it ensures internal forces between bodies cancel so total momentum is conserved) but is not the conservation law itself.
- The other two statements are incorrect: momentum need not always change (statement 2 is false) and an external net force is required to change the total momentum of a system (statement 3 is false).