A long, solid, non-conducting cylinder of radius \(R\) carries a volume charge density given by \(\rho(r) = \rho_0 \dfrac{r}{R}\) for \(r \le R\), where \(\rho_0\) is a positive constant and \(r\) is the radial distance from the central axis of the cylinder. Which of the following expressions gives the magnitude of the electric field at a distance \(r < R\) from the central axis?