c
$y =\frac{\text { stress }}{\text { strain }}=2.0 \times 10^{10}$
Energy density $=\frac{1}{2} \; stress \; \times \; strain$
$=\frac{1}{2}(\text { strain })^{2} y =\frac{1}{2}\left(5 \times 10^{-4}\right)^{2} \times 20 \times 10^{10}$
$=25 \times 10^{2} \times 10=25 \frac{ kJ }{ m ^{3}}$
Ans. $25$