Answer
Slight changes in position of the fifth charge will disrupt the equilibrium.
The equilibrium is unstable.
Work Step by Step
The magnitude of the electrostatic force is given by 19-5,
$F =k\displaystyle \frac{|q_{1}||q_{2}|}{r^{2}}$
(Force is inversely proportional to the square of the distance between the charges.)
A slight change in position of the fifth charge would change the magnitudes (and directions) of the individual electrostatic forces acting on it, so, unless we perform meticulous calculations on where this new position of equilibrium might be,
in general, a net force will exist, and the fifth charge will accelerate away from the point of equilibrium.
