Final answer:
The correct answer is option 1) friction.
Step-by-step explanation:
The momentum of a roller skate is not conserved because friction acts on the skates. According to Newton's First Law of Motion, in an ideal frictionless environment, an object will continue to move indefinitely without changing its speed or direction, which demonstrates the concept of inertia.
However, friction is a nonconservative force that acts in the opposite direction of motion, hence slowing down the roller skate. Therefore, when a roller skate rolls along the ground, friction between the skate and the ground exerts a force that opposes the motion, leading to a change in momentum.
For example, as seen in the action between the tires of a car and the road, friction exerts a horizontal force that changes the car's momentum. Moreover, even when two people apply the same force to throw two identical balls in the air, these balls will not necessarily travel the same distance because different frictional forces may be acting on them.