A non-linear dielectric material placed between parallel conducting plates has an electric displacement field magnitude given by \(D = \varepsilon_0 E + \beta E^2\), where \(E\) is the electric field magnitude and \(\beta\) is a positive constant. The electrostatic energy density stored in the material is defined by the work required per unit volume to establish the field, given by \(u = \int_0^E E' \, dD'\). Which of the following expressions represents the electrostatic energy density \(u\) stored in the dielectric?