c
(c)
Work done = energy stored
$W=\frac{1}{2} \times \text { force } \times \text { elongation }$ $\left\{\right.$ Force $=\frac{\Delta L}{L} \cdot A Y$
$W=\frac{1}{2} \times \frac{\Delta L}{L} \times A \times Y \times \Delta L$
$W=\frac{1}{2} \frac{A Y}{L} \times \Delta L^2$
$W \propto \Delta L^2$