A: When currents vary with time, Newton's third law is valid only if momentum carried by the electromagnetic field is taken into account.
R: Magnetic field lines always form closed loops.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion then mark (1)
B. If both Assertion & Reason are true but the reason 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 assertion and reason question, let's break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "when currents vary with time, Newton's third law is valid only if momentum carried by the electromagnetic field is taken into account."

- **Explanation**: Newton's third law states that for every action, there is an equal and opposite reaction. In the context of varying currents, the electromagnetic field created by the current also carries momentum. Therefore, when analyzing forces and interactions in systems with time-varying currents, we must consider the momentum of the electromagnetic field to fully satisfy Newton's third law.

### Step 2: Understand the Reason
The reason states that "magnetic field lines always form closed loops."

- **Explanation**: This is a fundamental property of magnetic fields. Magnetic field lines emerge from the north pole of a magnet and enter the south pole, forming closed loops. This characteristic is essential in understanding how magnetic fields interact with electric currents.

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

- While both statements are true, the reason does not explain why the assertion is valid. The assertion is about the necessity of considering the momentum of the electromagnetic field when currents change, while the reason simply describes a property of magnetic field lines.

### Conclusion
Based on the analysis:
- **Assertion**: True
- **Reason**: True
- **Relationship**: The reason does not explain the assertion.

Thus, the correct answer is that both the assertion and reason are true, but the reason is not a correct explanation for the assertion.

### Final Answer
Both assertion and reason are true, but the reason is not a correct explanation for the assertion.

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