Final answer:
The principle described in the question aligns with the Law of Conservation of Momentum, which states that the total momentum of an isolated system remains constant provided no external forces act upon it.
Step-by-step explanation:
The principle that states in an isolated system the total momentum before any event is equal to the total momentum after the event is known as the Law of Conservation of Momentum, not explicitly one of Newton's Laws. However, this principle is a consequence of Newton's laws, particularly relating to Newton's Third Law and a general understanding of Newton's Second Law when applied to a system of objects. If we narrow down the options provided, the correct answer is (d) Law of Conservation of Momentum. This states that momentum is conserved for an isolated system with any number of objects in it, meaning no matter how many objects interact within that system, their total momentum remains unchanged provided no external forces are acting upon them.