Assertion: Length of the metal rod is aligned along northsouth direction and is released from rest under gravity. Potential difference is generated along the length of the rod.
Reason: When a metal rod is moved perpendicular to its length and parallel to magnetic field then magnetic force on electrons generates electric field along the length of rod.

Options

A. If both assertion and reason are correct and reason is correct explanation of the assertion
B. If both assertion and reason are correct but reason is not correct explanation of the assertion
C. If assertion is correct but reason is incorrect.
D. If both assertion and reason are incorrect

Answer

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

### Step 1: Analyze the Assertion
The assertion states that a metal rod aligned along the north-south direction, when released from rest under gravity, generates a potential difference along its length.

- **Understanding the Assertion**: The assertion implies that as the rod falls, it will create a potential difference due to its motion in the Earth's magnetic field. However, the Earth's magnetic field lines run from the south to the north. When the rod falls vertically (along the direction of gravity), it does not move perpendicular to the magnetic field lines. Instead, it moves parallel to them.

- **Conclusion for Assertion**: Since the rod is falling along the magnetic field lines and not cutting across them, it does not induce a potential difference along its length. Therefore, the assertion is incorrect.

### Step 2: Analyze the Reason
The reason states that when a metal rod is moved perpendicular to its length and parallel to the magnetic field, the magnetic force on electrons generates an electric field along the length of the rod.

- **Understanding the Reason**: This statement discusses the conditions under which an electric field is generated in a rod. For an electric field to be induced, the rod must move in a direction that is perpendicular to both its length and the magnetic field lines.

- **Conclusion for Reason**: However, the reason incorrectly states that the rod is moving parallel to the magnetic field while being perpendicular to its length. If the rod is parallel to the magnetic field, then the angle between the velocity vector and the magnetic field vector is zero, resulting in no magnetic force acting on the electrons (as the Lorentz force is given by \( F = q(\mathbf{v} \times \mathbf{B}) \)). Therefore, the reason is also incorrect.

### Final Conclusion
Both the assertion and the reason are incorrect.

### Answer
The correct option is: **4. Both Assertion and Reason are incorrect.**

---