JEE Main · 2015 · 04 Apr 2015 · Offline · Magnetic Effects of Current

A current loop feels a torque $\vec{\tau }=I\vec{A}\times \vec{B}$ and stores energy $U=-I\vec{A}\cdot \vec{B}$, so it rests in equilibrium only when its area vector lines up with the field, parallel or antiparallel. Among the orientations drawn, those are the two whose normal points along the $z$-axis. With the area vector along $+\hat{z}$, parallel to the field, the energy is least, so orientation (b) is the stable one; with it along $-\hat{z}$, antiparallel, the energy is greatest, so orientation (d) is in unstable equilibrium.