(d) $Y = \frac{F/A}{\mathrm{\Delta}l/l}$
Given, $F/A =$ stress $= 3.18 \times 10^{8}Nm^{- 2}$
$l = 1m,\ Y = 2 \times 10^{11}Nm^{- 2}$
$\mathrm{\Delta}l = \frac{lF/A}{Y} = \frac{1 \times 3.18 \times 10^{8}}{2 \times 10^{11}} = 1.59 \times 10^{- 3}m = 1.59mm$