Assertion: Diamagnetism is universal; it is present in all materials.
Reason: Field due to induced magnetic moment is opposite to magnetizing field.
A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true and reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Hint: Diamagnetism is a weak form of magnetism which arises only when an external (magnetizing) field is applied. It is induced by change in orbital motion of electrons due to applied magnetic field.

Complete answer:
All magnetism basically can be thought to arise due to charges in motion. Inside an atom electrons are the charges that are under constant motion. An electron has an orbital motion about the nucleus and also can be thought to have a spin motion like earth.
Most dominant magnetism- ferromagnetism arises due to alignment of spins in a material leading to formation of domains. Besides that, paramagnetism is observed when there is incomplete cancellation of spins of electrons inside the atom. Whereas, diamagnetism is observed when there is complete cancellation of spins. Diamagnetic substances are repelled by magnetic fields i.e., when kept in a magnetic field, diamagnetic substances are attracted to weaker sections of the field.

Since in general all the atoms possess electrons having some orbital motion, diamagnetism is a property related to all materials. Therefore, the assertion and the reason are both correct statements but the reason is not good enough to explain the assertion.

So, the correct answer is “Option B”.

Note:
The reason given here is trying to imply that since repulsion is observed in diamagnetic materials it should be associated with all the materials. We already know that not all the materials show repulsion to the magnetic field. Therefore it is easy to conclude that the two statements do not fit together to give a flawless logic.