c
Energy per unit volume \(=\frac{1}{2} \text { stress } \times \text { strain }\)

Energy \(=\frac{1}{2} \text { stress } \times \text { strain } \times \text { volume }\)

\(80=\frac{1}{2} \times Y \times \text { strain }^2 A \times \ell\)

\(80=\frac{1}{2} \times 2 \times 10^{11} \times \frac{\left(2 \times 10^{-2}\right)^2}{400} \times A \times 20\)

\(20=\frac{10^{+\prime}}{20} \times A\)

\(40 \times 10^{-6} m ^2=A\)

\(A=40\,mm ^2\)