A: Above curie-point a ferromagnetic substance behaves as a paramagnetic substance.
R: Magnetic susceptibility of a diamagnetic substance increases with rise in temperature.

Options

A. If both Assertion & Reason are the true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reson is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the given assertion and reason question, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
The assertion states: "Above Curie-point a ferromagnetic substance behaves as a paramagnetic substance."

- **Explanation**: The Curie point (or Curie temperature) is the temperature above which a ferromagnetic material loses its ferromagnetic properties and becomes paramagnetic. This occurs because the thermal energy at temperatures above the Curie point disrupts the alignment of magnetic dipoles in the material. In a ferromagnetic material, the magnetic moments are aligned in the same direction, but when heated above the Curie point, this alignment is lost, and the material behaves like a paramagnet, where the magnetic moments are randomly oriented.

### Step 2: Analyze the Reason
The reason states: "Magnetic susceptibility of a diamagnetic substance increases with rise in temperature."

- **Explanation**: Magnetic susceptibility (χ) of diamagnetic materials is generally negative and does not increase with temperature. In fact, it tends to decrease with an increase in temperature. This is because the induced magnetic moment in diamagnetic materials arises from the orbital motion of electrons, which is affected by temperature. As temperature increases, the thermal agitation of the electrons increases, leading to a decrease in the induced magnetic moment and thus a decrease in magnetic susceptibility.

### Conclusion
- The assertion is **true** because a ferromagnetic substance does behave like a paramagnetic substance above the Curie point.
- The reason is **false** because the magnetic susceptibility of a diamagnetic substance does not increase with temperature; it decreases.

### Final Answer
The correct option is that the assertion is true, but the reason is false.