A particle of mass \(m\) moves along the \(x\)-axis under the influence of a conservative net force given by \(F(x) = -kx + \beta x^3\), where \(k\) and \(\beta\) are positive constants. The potential energy of the system is defined such that \(U(0) = 0\). Which of the following graphs best represents the potential energy \(U(x)\) as a function of position \(x\)?