The spring potential energy $US$ is $($a$)$ zero at $y=y_0+A.($b$)$ negative at $y=y_0-A.($c$)$ maximum of $\frac{1}{2}k{A}^2$ at $y=y_0+$A and $y=y_0-A.($d$)$ maximum of $\frac{1}{2}k{A}^2$ at $y=y_0.$