d
For stable equilibrium potential energy should be minimum
$\mathrm{U}=-\mathrm{MB} \cos \theta$
$\theta \rightarrow$ angle $\mathrm{b} / \mathrm{w}\,\,\, \overrightarrow{\mathrm{M}}$ and $\overrightarrow{\mathrm{B}}$
$\mathrm{U}_{\min }$ at $\theta=0^{\circ}$