a particle B of charge Q and mass m is in equilibrium under weight and electrostatics force applied by a fixed charge A, which is directly beneath the particle B as shown in the figure. when particle B is disturbed along vertical , the equilibrium is1. Stable 2. Unstable 3. Neutral 4. There can not be in equilibrium

We can notice that the weight of the particle would never change, so the force due to its weight will be constant. The deciding factor will be the electrostatic force. When we take the particle upwards, the upward electrostatic force will reduce and hence the particle will try to move towards the equilibrium point. When it goes downwards the electrostatic force will increase and hence it will try to move upwards that is towards it equilibrium position. Hence, by definition the point where it was initially at will be the point of stable equilibrium.