c
Energy per unit volume $=\frac{1}{2} \text { stress } \times \text { strain }$
Energy $=\frac{1}{2} \text { stress } \times \text { strain } \times \text { volume }$
$80=\frac{1}{2} \times Y \times \text { strain }^2 A \times \ell$
$80=\frac{1}{2} \times 2 \times 10^{11} \times \frac{\left(2 \times 10^{-2}\right)^2}{400} \times A \times 20$
$20=\frac{10^{+\prime}}{20} \times A$
$40 \times 10^{-6} m ^2=A$
$A=40\,mm ^2$