A solid cylinder of mass \(M\) and radius \(R\) has a non-uniform volumetric mass density that varies linearly with radial distance \(r\) from its central axis according to \(\rho(r) = Cr\), where \(C\) is a constant. The cylinder rolls without slipping down a ramp inclined at an angle \(\theta\) to the horizontal. Which of the following expressions represents the linear acceleration of the center of mass of the cylinder as it rolls down the incline?