A long, solid insulating cylinder of radius \(R\) has 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. The cylinder is coaxially surrounded by a thin conducting cylindrical shell of radius \(3R\). Which of the following expressions gives the electric field magnitude \(E(r)\) as a function of distance \(r\) in the region \(R < r < 3R\)?