$\mathrm{l}=\frac{\mathrm{FL}}{\mathrm{AY}}\therefore \mathrm{l}\propto \frac{1}{{\mathrm{r}}^2}(\mathrm{Y},\mathrm{L}\text{ and }\mathrm{F}\text{ are constant)}$

i.e. for the same load, the thickest wire will show minimum elongation. So, graph D represents the thickest wire.