A stepped pulley consisting of two concentric, rigidly attached cylindrical sections is mounted on a fixed, horizontal frictionless axle through its center. The entire pulley has a total rotational inertia \(I\). A light string is wrapped securely around the inner cylinder of radius \(r\) and attached to a block of mass \(m\), while the outer cylinder of radius \(R\) remains free. The system is released from rest, and the string unwinds without slipping as the block accelerates downward. Which of the following expressions represents the magnitude of the linear acceleration \(a\) of the block in terms of \(m\), \(r\), \(I\), and \(g\)?