Final answer:
Most moving objects slow down and stop due to frictional forces, which are external forces acting in opposition to motion, leading to the non-conservation of momentum in a non-isolated system.
Step-by-step explanation:
Most moving objects eventually slow down and stop due to frictional forces, which is the correct answer. According to Newton's first law of motion, a body in motion will continue in motion at a constant velocity unless acted upon by a net external force. Friction is an external force that acts opposite to the direction of motion, causing objects to slow down and eventually stop. Momentum is conserved in a system when no net external forces act on the system, meaning that the total momentum before an interaction is equal to the total momentum after the interaction as long as the system is isolated from external influences.