Correct option is
B $\frac{1(\text { stress })^{2}}{2}$
The elastic energy stored per unit volume in a stretched wire is
$$
\mathrm{u}=\frac{1}{2} \times \text { stress } \times \operatorname{strain}=\frac{(\text { stress })^{2}}{2 \mathrm{Y}}\left(\because \mathrm{Y}=\frac{\text { stress }}{\text { strain }}\right)
$$