Option(a) is correct because

Magnetic moments are induced in all materials when a magnetic field is applied. Diamagnetism is a property which states that resultant field in such materials are smaller than the applied field.

Therefore, diamagnetism is exhibited by all materials.

Option(b) is incorrect as diamagnetism results from induction of dipole moments in the atoms in presence of applied field and not due to partial alignment of permanent magnetic moment.

Option(c) is incorrect because the magnetizing field intensity is given by formula

In free space (vacuum) also magnetic field exists and μ₀ is given by

4π × 10^-7henry per meter

Option(d) is correct because of lenz’s law magnetic field of induced magnetic moment is always opposite to the applied field.

Lenz’s Law:

Direction of current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.

Therefore, the magnetic field of induced magnetic moment is opposite to the applied field.

Thus, option (a) and (d) are correct option.