Asertion: magnetic field due to a infinite stragith conductor varies inversely as the distance from it.
Reason: the magentic field at the centre of the circular coil is zero

Options

A. If both assertion and Reason are true and the Reason is the correct explanation of the assertion.
B. If both assertion and Reason are true but the reason is not the correct explanation of the 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 reason provided regarding the magnetic field due to an infinite straight conductor and the magnetic field at the center of a circular coil.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that the magnetic field due to an infinite straight conductor varies inversely as the distance from it.

- **Mathematical Expression**: According to the Biot-Savart law, the magnetic field \( B \) at a distance \( R \) from a straight conductor carrying current \( I \) is given by:
\[ B = \frac{\mu_0 I}{2 \pi R} \]
- Here, \( \mu_0 \) is the permeability of free space. From this equation, it is clear that \( B \) is inversely proportional to \( R \). Thus, the assertion is **true**.

2. **Understanding the Reason**:
The reason states that the magnetic field at the center of a circular coil is zero.

- **Magnetic Field at the Center of a Circular Coil**: For a circular coil of radius \( R \) carrying a current \( I \), the magnetic field at the center is given by:
\[ B = \frac{\mu_0 I}{2R} \]
- This expression shows that the magnetic field at the center of the coil is not zero; it is a non-zero value depending on the current and the radius of the coil. Therefore, the reason is **false**.

3. **Conclusion**:
Since the assertion is true and the reason is false, we can conclude that the correct relationship is:
- Assertion: True
- Reason: False

### Final Answer:
The assertion is true, but the reason is false.