b
Elastic energy $=\frac{Y}{2}(\text { strain })^{2} \times$ Area $\times$ length
Elastic energy per unit length $=\frac{Y}{2}(\text { strain })^{2} \times$ Area
$\left(\text { strain }=\frac{\Delta \ell}{\ell}=\alpha \Delta T=10^{-5} \times 10=10^{-4}\right)$
$=\frac{10^{11}}{2} \times\left(10^{-4}\right)^{2} \times 10^{-2}=5\, {J} / {m}$