a
In figure no. \((a)\) and \((c)\), a constant force equal to mg sinq is required. After reaching the highest point, in case of figure \((c)\), no force is required but in case of figure \((a)\), body travels on its own. So a \(-ve\) force is acting on the body. In this way, figure \((a)\) represents the given \(F -x\) curve.