The force acting on a body moving along -axis varies with the position of the particle as shown in the fig

The body is in stable equilibrium at

a)

b)

c)

Both  and

d)

Neither  nor

(b)

When particle moves away from the origin then at position  force is zero and at , force is positive (repulsive in nature) so particle moves further and does not return back to original position

 the equilibrium is not stable

Similarly at position  force is zero and at , force is negative (attractive in nature)

So particle return back to original position  the equilibrium is stable