Assertion:Iron, cobalt, nickel and `CrO_2` are called ferromagnetic substances .
Reason:Ferromagnetic substances are weakly attracted by magnetic field

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but 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

Explanation

To solve the question, we need to evaluate the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Iron, cobalt, nickel, and `CrO_2` are called ferromagnetic substances."

- **Explanation**: Ferromagnetic substances are materials that can be magnetized; they have a high susceptibility to magnetization and retain their magnetic properties even after the external magnetic field is removed. Iron, cobalt, nickel, and chromium dioxide (`CrO_2`) are indeed known ferromagnetic materials.

### Step 2: Analyze the Reason
The reason states that "Ferromagnetic substances are weakly attracted by magnetic field."

- **Explanation**: This statement is incorrect. Ferromagnetic substances are not weakly attracted; rather, they are strongly attracted to magnetic fields. When exposed to a magnetic field, the magnetic domains within these materials align in the direction of the field, resulting in a strong attraction.

### Step 3: Conclusion
Based on the analysis:
- The assertion is true: Iron, cobalt, nickel, and `CrO_2` are ferromagnetic substances.
- The reason is false: Ferromagnetic substances are strongly attracted by magnetic fields, not weakly.

Thus, the correct answer is that the assertion is true, but the reason is false.

### Final Answer
The correct option is: Assertion is true, but reason is false.

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