Four identical, positive, point charges of magnitude Q are placed at the vertices of a square of side 2d. What is the electric potential produced at the centre of the square by the four charges?

A.     0

B.     \(\frac{{4kQ}}{d}\)

C.     \(\frac{{\sqrt 2 kQ}}{d}\)

D.     \(\frac{{2\sqrt 2 kQ}}{d}\)

D