To find the electric potential at the center of the square ABC, we can use the principle of superposition, which states that the total potential is the sum of the potentials due to each individual charge.

Let's assume that the side length of the square is "a" and the charges at each corner are all positive and have the same magnitude "Q".

The electric potential due to a point charge at a distance "r" is given by the equation:

V = k * Q / r

where V is the potential, k is the electrostatic constant (k = 8.99 * 10^9 Nm^2/C^2), Q is the charge, and r is the distance from the charge to the point where the potential is being measured.

In this case, since all four charges are equidistant from the center of the square, the distances from the center to each charge are all equal. Let's call this distance "d".

The potential at the center due to each charge is then:

V1 = k * Q / d
V2 = k * Q / d
V3 = k * Q / d
V4 = k * Q / d

To find the total potential at the center, we sum the potentials due to each charge:

V_total = V1 + V2 + V3 + V4
= k * Q / d + k * Q / d + k * Q / d + k * Q / d
= 4 * k * Q / d

Therefore, the electric potential at the center of the square ABC is 4 * k * Q / d.