Assertion: Two identical solid balls, one of ivory and the other of wet clay are dropped from the same height on the floor. Both the balls will rise to the same height after.
Reason: Ivory and wet clay have the same elasticity.
A. Both assertion and reason are true and the reason is the correct explanation of the assertion.
B. Both assertion and reason are true but the reason is not the correct explanation of the assertion.
C. Assertion is true but the reason is false.
D. Assertion and reason both are false.

Hint: In order to answer this question it is good to know about a few things related to elasticity. It is the property of a material that regains its shape and size after the deformation force is removed.

Complete step by step solution:
When two identical solid balls, one made of ivory and the other of wet clay are dropped from the same height on the floor. Then the ivory ball will rise to a greater height and the ball which is made of wet clay will not rise to a greater height, it just flattened on the ground permanently.

Because the elasticity of the wet clay is less compared to the ivory ball i.e., the ivory ball is more elastic. So, compared to wet clay, the ivory ball rises to a greater height which means the ivory ball regains its original shape after striking the floor and the wet clay does not regain its original shape after reaching the floor.

Hence, option D is the correct answer.

Note: Most of the solid materials display some degree of elastic behaviour, but each material has a maximum amount of stress and deformation that it can withstand before it cannot recover elastically. This cap, also known as the elastic cap.