An irregularly shaped spool of total mass \(M\) and outer radius \(R\) rolls without slipping down an inclined track from rest. A student releases the spool from several different vertical heights \(h\) above a photogate at the base of the track, which measures the translational speed \(v\) of the spool. A plot of \(v^2\) on the vertical axis versus \(h\) on the horizontal axis yields a straight line with slope \(S\). In terms of \(S\), \(M\), \(R\), and acceleration due to gravity \(g\), which of the following is the rotational inertia \(I\) of the spool about its central axis?