1. Magnetic dipole moment of the coil

For a circular coil of radius $r$, area is $A = \pi r^2$

If the coil has $N$ turns and carries current $I$, its magnetic moment is $\vec m = N I A \, \hat n = N I \pi r^2 \, \hat n$ where $\hat n$ is normal to the plane of the coil.

2. Direction of magnetic moment

The coil is held in the XZ-plane. So its normal is along the $Y$-axis. Hence $\vec m$ is along $\pm \hat j$.

Given magnetic field is $\vec B = B\hat i$ which is along the $X$-axis.

3. Angle between $\vec m$ and $\vec B$

Since $\vec m$ is along $\hat j$ and $\vec B$ is along $\hat i$, the angle between them is $\theta = 90^\circ$

4. Torque on a current loop

Magnitude of torque is $\tau = mB\sin\theta$

Substituting: $\tau = (N I \pi r^2)B\sin 90^\circ$ $\tau = N I \pi r^2 B$

5. Match with options

Thus, $\boxed{\tau = B\pi r^2 I N}$

This matches Option B.