Energy density =$\large\frac{1}{2}$$\times $stress$\times$strain
$\Rightarrow \large\frac{1}{2}\times \frac{f}{a}\times \frac{\Delta l}{l}$
$\Rightarrow \large\frac{1}{2}\times \frac{10^5}{2\times 10^{-4}}\times \frac{10^{-2}}{2}$
$\Rightarrow \large\frac{1}{8}$$\times 10^{-1}$
$\Rightarrow 3.125\times 10^{-3}J$
$\Rightarrow 3.125mJ$
Hence (B) is the correct answer.