A solid insulating sphere of radius \(R\) contains a total net charge \(Q\). The volume charge density \(\rho\) inside the sphere varies directly with radial distance \(r\) from the center, such that \(\rho(r) = C r\) for \(r \le R\), where \(C\) is a constant. In terms of \(Q\), \(R\), \(r\), and fundamental constants, which of the following expressions represents the magnitude of the electric field at a distance \(r < R\) from the center of the sphere?