A solid nonconducting sphere of radius \(R\) has a non-uniform volume charge density given by \(\rho(r) = \rho_0 \dfrac{r}{R}\) for \(r \le R\), where \(\rho_0\) is a constant, and a total net charge \(Q\). The sphere is assembled by bringing infinitesimal charge elements from infinity and depositing them layer by layer in concentric spherical shells.

Which of the following expressions represents the total work required by an external agent to assemble this sphere?