A charge q is placed at the centroid of an equilateral triangle. Three charges equal to Q are placed at the vertices of the triangle . The system of the four charges will be in equilibrium . if q is equal to

Question

$\begin{array}{1 1} -Q \sqrt {3} \\ \large\frac{-Q}{3} \\ \large\frac{-Q}{\sqrt 3} \\ \large\frac{Q}{\sqrt 3} \end{array} $