An isolated parallel-plate capacitor with square plates of side length \(L\) and plate separation \(d\) holds a fixed positive charge \(Q\). A dielectric slab with dielectric constant \(\kappa\) and thickness \(d\) is inserted a distance \(x\) into the capacitor (\(0 < x < L\)). Let \(C_0 = \dfrac{\varepsilon_0 L^2}{d}\) be the capacitance of the empty capacitor. Which of the following expressions gives the magnitude of the electrostatic force exerted on the dielectric slab by the capacitor's electric field as a function of \(x\)?