Question 31:
easy9. Point charges $+4q$, $-q$ and $+4q$ are kept on the X-axis at point $x = 0$, $x = a$ and $x = 2a$ respectively. (1988)
The net force on each charge is zero, so all are in equilibrium. However, displacing any charge slightly along the axis results in a net force acting in the direction of displacement, meaning the equilibrium is unstable for all (Earnshaw's theorem).