Conservation of Momentum Principle

Steps:

• Recall that conservation of momentum states the total momentum in a closed system remains unchanged without external forces.
• Evaluate each option against this definition.
• Eliminate options describing related concepts like force or elastic collisions.
• Select the option matching the exact principle.

Why D is correct:

• It directly states the law: total momentum of a system is constant if no net external force acts, as per Newton's first law extended to systems.

Why the others are wrong:

• A describes Newton's second law (F = dp/dt).
• B incompletely defines elastic collisions, focusing only on relative momentum.
• C defines momentum (p = mv), not its conservation.

Final answer: D