Assertion: The ferromagnetic substance do not obey Curie's law.
Reason: At Curie point a ferromagnetic substance start behaving as a paramagnetic subsrance.

Options

A. If both Assertain and Reason are true and Reason is the correct explanation of Assertain
B. If both Assertain and Reason are true but Reason is not correct explantion of Assertain
C. If Assertion is true but Reason is false
D. If Assertion is false but Reason in true.

Explanation

To solve the assertion-reason question, we will break down the concepts involved and analyze the statements step by step.

### Step 1: Understand the Assertion
The assertion states that "ferromagnetic substances do not obey Curie's law."

**Explanation:**
Curie's law applies to paramagnetic materials, which states that the magnetic susceptibility of a paramagnetic substance is inversely proportional to its temperature. However, ferromagnetic substances exhibit a different behavior below their Curie temperature (Tc). Below Tc, they have a high magnetic susceptibility and do not follow Curie’s law. Therefore, the assertion is true.

### Step 2: Understand the Reason
The reason states that "at the Curie point, a ferromagnetic substance starts behaving as a paramagnetic substance."

**Explanation:**
At the Curie temperature (Tc), ferromagnetic materials lose their ferromagnetic properties and transition to a paramagnetic state. This means that above the Curie point, they begin to obey Curie's law, as they behave like paramagnetic substances. Hence, this statement is also true.

### Step 3: Analyze the Relationship Between Assertion and Reason
Now we need to determine if the reason correctly explains the assertion.

**Explanation:**
While both the assertion and reason are true, the reason does not serve as a correct explanation for the assertion. The assertion claims that ferromagnetic substances do not obey Curie's law, which is true only below the Curie point. The reason explains the behavior at the Curie point but does not clarify why ferromagnetic substances do not obey Curie's law in general.

### Conclusion
Based on the analysis:
- The assertion is true.
- The reason is true but does not explain the assertion.

Thus, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion. This matches with option B.