To solve this problem we will apply the concept of voltage given by Coulomb's laws. From there we will define the charges and the distance, and we will obtain the total value of the potential difference in the system.
The length of diagonal is given as

The distance of the center of the square from each of the corners is

The potential electric at the center due to each cornet charge is




The total electric potential at the center of the given square is


Al the charges are equal, and the distance are equal to a, then


Therefore the correct option is E.