Analyze Statement I

Statement I: Susceptibilities of paramagnetic and ferromagnetic substances increase with decrease in temperature.

• For paramagnetic substances, magnetic susceptibility follows Curie's law: $\chi = \frac{C}{T}$ where $C$ is Curie's constant and $T$ is absolute temperature.

  Hence, as $T$ decreases, $\chi$ increases.

• For ferromagnetic substances, above Curie temperature they follow the Curie-Weiss law: $\chi = \frac{C}{T-\theta}$ where $\theta$ is the Weiss constant. This also shows susceptibility increases as temperature decreases (up to the relevant regime).

Therefore, Statement I is true.

Analyze Statement II

Statement II: Diamagnetism is a result of orbital motions of electrons developing magnetic moments opposite to the applied magnetic field.

• In diamagnetic materials, there is no permanent magnetic dipole moment in the absence of field.
• When an external magnetic field is applied, the orbital motion of electrons is modified, inducing magnetic moments.
• By Lenz's law, these induced magnetic moments oppose the applied magnetic field.

Therefore, Statement II is true.

Evaluate the options

• A: Both Statement I and Statement II are true. ✅
• B: Both Statement I and Statement II are false. ❌
• C: Statement I is true but Statement II is false. ❌
• D: Statement I is false but Statement II is true. ❌

Final Answer

The correct option is: $\boxed{A}$

Comparison with stored correct answer

Stored correct answer: A

My derived answer matches the stored answer.