Four point charges are fixed at the corners of a square of side length \(d\), as shown in the figure. The top-left and top-right corners each hold a charge of \(+q\), while the bottom-left and bottom-right corners each hold a charge of \(-q\). A small positive test charge \(+q_0\) is placed at the center of the square. Which of the following expressions represents the magnitude of the net electrostatic force exerted on the test charge?