Answer: The correct answer is Option D.
Explanation:
Law of conservation of energy states that energy cannot be created or destroyed, but is transformed from one form of energy to another form. The total energy of the system remains constant.
In the Newton's cradle, there is elastic collision between the balls. The energy at first was potential energy and it was transformed in the form of kinetic energy as the ball swing up.
In elastic collision, the energy and momentum remains conserved.
The energy possessed by the first ball was potential energy due to its position. On collision, the potential energy will be converted into kinetic energy and transferred to the second ball. The energy will be transferred to the rest of the balls in contact one by one till the last ball starts to move and kinetic energy transforms to potential energy again.
This process will continue until and unless stopped by an external unbalanced force. As we know, from the first law of motion an object continues to be in the state of rest or motion unless and until an external unbalanced force is applied on it.
Hence, the correct option is Option D.
