Q.A particle is placed at the origin and a force $F = -kx$ is acting on it (where $k$ is a positive constant). If $U(0) = 0$, the graph of $U(x)$ versus $x$ will be one of four labelled curve sketches (a)-(d). (IIT 2004) [Figure only: the four options are graph shapes with no distinguishing printed text, so the correct graph cannot be identified from the extracted text alone.]