A solid sphere of radius \(R\) and total mass \(M\) has a volume mass density that varies radially according to \(\rho(r) = \rho_0 \left(\dfrac{r}{R}\right)\), where \(\rho_0\) is a constant and \(r\) is the distance from the center of the sphere. Which of the following expressions represents the rotational inertia of the sphere about an axis passing through its center?