An insulating solid sphere of radius \(R\) has a non-uniform volume charge density given by \(\rho(r) = a r^2\) for \(0 \le r \le R\), where \(a\) is a positive constant and \(r\) is the radial distance from the center. The total charge of the sphere is \(Q\). Which of the following expressions gives the magnitude of the electric field \(E(r)\) inside the sphere at a distance \(r < R\) from the center?