An infinitely long, solid insulating cylinder of radius \(R\) carries a non-uniform volume charge density given by \(\rho(r) = \rho_0 \left(1 - \dfrac{r}{R}\right)\) for \(r \le R\), where \(\rho_0\) is a positive constant and \(r\) is the radial distance from the central axis. Which of the following expressions gives the magnitude of the electric field \(E(r)\) inside the cylinder as a function of \(r\) for \(r < R\)?