A non-uniform solid cone has total mass \(M\), height \(H\), and base radius \(R\). The volumetric mass density of the cone varies linearly with the distance \(z\) from the apex according to \(\rho(z) = Cz\), where \(C\) is a positive constant and \(0 \le z \le H\). Which of the following expressions represents the rotational inertia of the cone about its central axis of symmetry?