d
\(\frac{\text { Energystored }}{\text { Volume }}=\frac{1}{2} \frac{(\text { Stress })^{2}}{\mathrm{Y}}\)

\(\frac{\mathrm{u}_{1}}{\mathrm{u}_{2}}=\frac{1}{4} \Rightarrow 4 \mathrm{u}_{1}=\mathrm{u}_{2}\)

\(4 \frac{1}{2 \mathrm{Y}}\left[\frac{\mathrm{W} \cdot 4}{\pi \mathrm{d}_{1}^{2}}\right]^{2}=\frac{1}{2 \mathrm{Y}}\left[\frac{\mathrm{W} \cdot 4}{\pi \mathrm{d}_{2}^{2}}\right]^{2}\)

\(4=\left(\frac{\mathrm{d}_{1}}{\mathrm{d}_{2}}\right)^{4}\)

\(\Rightarrow \frac{\mathrm{d}_{1}}{\mathrm{d}_{2}}=\sqrt{2}: 1\)