In which of the following cases is the potential energy of the spring minimum?

Question

$\begin{array}{1 1} (a)\; \text{when it is compressed} \\ (b)\; \text{when it is extended} \\ (c)\; \text{when it is at its natural length} \\ (d)\; \text{when it is at its natural length but is kept at a height h above the ground } \end{array}$