Four point charges are fixed at the corners of a square with side length \(d\), as shown in the diagram. The charges at the top-left, top-right, bottom-right, and bottom-left corners are \(+2Q\), \(-Q\), \(-2Q\), and \(+Q\), respectively. A positive test charge \(+q\) is placed at the center of the square. Which of the following expressions represents the magnitude of the net electrostatic force acting on the test charge \(+q\)?