A short bar magnet of magnetic moment M is placed with its axis making an angle of 30^circ with a uniform magnetic field of magnitude B. The potentia…

The potential energy of a magnetic dipole in a uniform magnetic field is given by $U = -M \cdot B = -MB \cos\theta$. Substituting $\theta = 30^\circ$, we get $U = -MB \cos(30^\circ) = -MB (\sqrt{3}/2)$ which is incorrect; wait, $\cos(30^\circ) = \sqrt{3}/2$, so $U = -\frac{\sqrt{3}}{2}MB$. Let's re-verify options: option A is $-MB/2$ (corresponding to $60^\circ$), option B is $-\frac{\sqrt{3}}{2}MB$. Thus, correct answer is B.