A small square loop carries a current and hangs in a uniform magnetic field…

Step 1. Part (a). The forces on two opposite sides are equal and opposite, so the net force is zero. But the two forces act along different lines. They make a couple, and a couple gives a torque of size $mB\sin\theta$. A torque turns the loop without moving its centre.

Step 2. Part (b). Near a magnet the field is stronger at one side of the loop than at the other. The forces on the two sides are then not equal. They no longer cancel, so there is a net force as well as a torque. The loop moves bodily towards the stronger field when its moment points along the field.

Step 3. Part (c). The torque is zero when $\theta = 0^\circ$ or $180^\circ$, that is, when the moment points along the field or against it. The first position is stable. A small push brings the loop back. The second is unstable. A small push sends the loop away.