$\large\frac{\Delta l}{l}=\frac{2\pi r_2-2\pi r_1}{2\pi r_1}$
$\Rightarrow \large\frac{.8-.5}{.5}$
$\Rightarrow \large\frac{3}{5}$
As stress=Y$\times$ strain
$F=3\times 10^{10}\times \large\frac{3}{5}$$\times 2\times 10^{-6}$
$\;\;\;=\large\frac{18}{5}$$\times 10^4$
$\;\;\;=36KN$
Hence (B) is the correct answer.